For drm_gem_object_unreference callers are required to hold
dev->struct_mutex, which these paths don't. Enforcing this requirement
has become a bit more strict with

commit ef4c6270
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 15 09:36:25 2015 +0200

    drm/gem: Check locking in drm_gem_object_unreference

Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>