drm/gem: Check locking in drm_gem_object_unreference

Pretty soon only some drivers will need dev->struct_mutex in their
gem_free_object callbacks. Hence it's really important to make sure
everything still keeps getting this right.

v2: Don't check for locking before we check for non-NULL obj. Spotted
by Dan Carpenter.

Link: http://mid.gmane.org/1444894601-5200-10-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>