drm/i915: Hold an object reference whilst we shrink it

Whilst running the shrinker, we need to hold a reference as we unbind
the objects, or else we may end up waiting for and retiring requests,
which in turn may result in this object being freed.

This is very similar to the eviction code which also has to be very
careful to keep a reference to its objects as it retires and unbinds
them.

Another similarity, that Ben pointed out, is that as we may call
retire-requests, the unbound_list is outside of our control. We must
only process a single element of that list at a time, that is we can not
rely on the "safe" next pointer being valid after a call to
i915_vma_unbind().

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915]
  PGD 758d3067 PUD ac0d6067 PMD 0
  Oops: 0000 [#1] SMP
  Modules linked in: dm_mod snd_hda_codec_realtek iTCO_wdt iTCO_vendor_support pcspkr snd_hda_intel i2c_i801 snd_hda_codec snd_hwdep snd_pcm snd_page_alloc snd_timer snd lpc_ich mfd_core soundcore battery ac option usb_wwan usbserial uvcvideo videobuf2_vmalloc videobuf2_memops videobuf2_core videodev i915 video button drm_kms_helper drm acpi_cpufreq mperf freq_table
  CPU: 1 PID: 16835 Comm: fbo-maxsize Not tainted 3.11.0-rc7_nightlytop_8fdad4_20130902_+ #7977
  task: ffff8800712106d0 ti: ffff880028e4a000 task.ti: ffff880028e4a000
  RIP: 0010:[<ffffffffa0082892>]  [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915]
  RSP: 0018:ffff880028e4b9e8  EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffff880145734000 RCX: ffff880145735328
  RDX: ffff8801457353fc RSI: 0000000000000000 RDI: ffff88007597cc00
  RBP: ffff88007597cc00 R08: 0000000000000001 R09: ffff88014f257f00
  R10: ffffea0001d65f00 R11: 0000000000bba60b R12: ffff880149e5b000
  R13: ffff880145734001 R14: ffff88007597ccc8 R15: ffff88007597cc00
  FS:  00007ff5bc919740(0000) GS:ffff88014f240000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000008 CR3: 0000000028f4c000 CR4: 00000000001407e0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Stack:
   0000000000000000 ffff88007597cc00 ffff8801440d6840 0000000000000000
   ffff880145734000 ffffffffa007c854 0000000000000010 ffff88007597c900
   0000000000018000 00000000004a1201 ffff88007597cc60 ffffffffa007d183
  Call Trace:
   [<ffffffffa007c854>] ? i915_vma_unbind+0xe2/0x1d1 [i915]
   [<ffffffffa007d183>] ? __i915_gem_shrink+0xf1/0x162 [i915]
   [<ffffffffa007d2ee>] ? i915_gem_object_get_pages_gtt+0xfa/0x303 [i915]
   [<ffffffffa00795f4>] ? i915_gem_object_get_pages+0x54/0x89 [i915]
   [<ffffffffa007cbda>] ? i915_gem_object_pin+0x238/0x5ce [i915]
   [<ffffffff812cba5f>] ? __sg_page_iter_next+0x2b/0x58
   [<ffffffffa0082056>] ? gen6_ppgtt_insert_entries+0xf2/0x114 [i915]
   [<ffffffffa007fe4b>] ? i915_gem_execbuffer_reserve_vma.isra.13+0x79/0x18d [i915]
   [<ffffffffa008017c>] ? i915_gem_execbuffer_reserve+0x21d/0x347 [i915]
   [<ffffffffa0080bfb>] ? i915_gem_do_execbuffer.isra.17+0x4f3/0xe61 [i915]
   [<ffffffffa00795f4>] ? i915_gem_object_get_pages+0x54/0x89 [i915]
   [<ffffffffa007e405>] ? i915_gem_pwrite_ioctl+0x743/0x7a5 [i915]
   [<ffffffffa0081a46>] ? i915_gem_execbuffer2+0x15e/0x1e4 [i915]
   [<ffffffffa000e20d>] ? drm_ioctl+0x2a5/0x3c4 [drm]
   [<ffffffffa00818e8>] ? i915_gem_execbuffer+0x37f/0x37f [i915]
   [<ffffffff816f64c0>] ? __do_page_fault+0x3ab/0x449
   [<ffffffff810be3da>] ? do_mmap_pgoff+0x2b2/0x341
   [<ffffffff810e49be>] ? vfs_ioctl+0x1e/0x31
   [<ffffffff810e5194>] ? do_vfs_ioctl+0x3ad/0x3ef
   [<ffffffff810e5224>] ? SyS_ioctl+0x4e/0x7e
   [<ffffffff816f88d2>] ? system_call_fastpath+0x16/0x1b
  Code: 52 0c a0 48 c7 c6 22 30 0d a0 31 c0 e8 ef 00 f9 ff bf c6 a7 00 00 e8 90 5d 24 e1 f6 85 13 01 00 00 10 75 44 48 8b 85 18 01 00 00 <8b> 50 08 48 8b 30 49 8b 84 24 88 02 00 00 48 89 c7 48 81 c7 98
  RIP  [<ffffffffa0082892>] i915_gem_gtt_finish_object+0x68/0xbd [i915]
  RSP <ffff880028e4b9e8>
  CR2: 0000000000000008

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=68171
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
[danvet: Bikeshed the comments a bit as discussed with Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>