drm/i915: fix scheduling while holding the new active list spinlock

regression caused by commit 5e118f4139feafe97e913df67b1f7c1e5083e535:
i915_gem_object_move_to_inactive() should be called in task context,
as it calls fput();

Signed-off-by: Shaohua Li<shaohua.li@intel.com>
[anholt: Add more detail to the comment about the lock break that's added]
Signed-off-by: Eric Anholt <eric@anholt.net>

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge