drm: Use kref_put_mutex in drm_gem_object_unreference_unlocked

If kref_put_mutex returns true then the caller or the put function is responsible
for unlocking the mutex. The usual pattern assumes that the free
callback unlocks the mutex, but since that is shared with the locked
variant we need to explicitly unlock here.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>