drm/i915: Increase fb alignment to 64k

An untiled framebuffer must be aligned to 64k. This is normally handled
by intel_pin_and_fence_fb_obj(), but the intelfb_create() likes to be
different and do the pinning itself. However, it aligns the buffer
object incorrectly for pre-i965 chipsets causing a PGTBL_ERR when it is
installed onto the output.

Fixes:
  KMS error message while initializing modesetting -
  render error detected: EIR: 0x10 [i915]
  http://bugs.freedesktop.org/show_bug.cgi?id=22936

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Signed-off-by: Eric Anholt <eric@anholt.net>