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 <daniel.vetter@ffwll.ch>
Date:   Thu Oct 15 09:36:25 2015 +0200

    drm/gem: Check locking in drm_gem_object_unreference

Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/udl/udl_fb.c

@@ -536,7 +536,7 @@ static int udlfb_create(struct drm_fb_helper *helper,
 out_destroy_fbi:
 	drm_fb_helper_release_fbi(helper);
 out_gfree:
-	drm_gem_object_unreference(&ufbdev->ufb.obj->base);
+	drm_gem_object_unreference_unlocked(&ufbdev->ufb.obj->base);
 out:
 	return ret;
 }

drivers/gpu/drm/udl/udl_gem.c

@@ -52,7 +52,7 @@ udl_gem_create(struct drm_file *file,
 		return ret;
 	}
 
-	drm_gem_object_unreference(&obj->base);
+	drm_gem_object_unreference_unlocked(&obj->base);
 	*handle_p = handle;
 	return 0;
 }