drm/i915: Hold the spinlock whilst resetting unpin_work along error path

Delay taking the mutex until we need to and ensure that we hold the
spinlock when resetting unpin_work on the error path. Also defer the
debugging print messages until after we have released the spinlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>