drm/udl: Use unlocked gem unreferencing

drm/udl: Use unlocked gem unreferencing

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 ef4c6270bf2867e2f8032e9614d1a8cfc6c71663
Author: Daniel Vetter <[email protected]>
Date:   Thu Oct 15 09:36:25 2015 +0200

    drm/gem: Check locking in drm_gem_object_unreference

Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Dave Airlie <[email protected]>