drm/gem: Use container_of in drm_gem_object_free

Just a random thing I spotted while reading code - better safe than
sorry.

Link: http://mid.gmane.org/1444894601-5200-11-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>