drm/gem: Be more friendly with locking checks

BUG_ON kills the driver, WARN_ON is much friendlier. And usually nothing
bad happens when the locking is slightly busted.

v2: Fix typos in commit message Thierry spotted.
Reviewed-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_gem.c

@@ -766,7 +766,7 @@ drm_gem_object_free(struct kref *kref)
 	struct drm_gem_object *obj = (struct drm_gem_object *) kref;
 	struct drm_device *dev = obj->dev;
 
-	BUG_ON(!mutex_is_locked(&dev->struct_mutex));
+	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
 
 	if (dev->driver->gem_free_object != NULL)
 		dev->driver->gem_free_object(obj);