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

regression caused by commit 5e118f41:
i915_gem_object_move_to_inactive() should be called in task context,
as it calls fput();

Signed-off-by: Shaohua <Li&lt;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>