drm/cma-helper: Don't grab dev->struct_mutex for in mmap offset ioctl

Since David Herrmann's mmap vma manager rework we don't need to grab
dev->struct_mutex any more to prevent races when looking up the mmap
offset. Drop it and instead don't forget to use the unref_unlocked
variant (since the drm core still cares).

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_gem_cma_helper.c

@@ -289,20 +289,15 @@ int drm_gem_cma_dumb_map_offset(struct drm_file *file_priv,
 {
 	struct drm_gem_object *gem_obj;
 
-	mutex_lock(&drm->struct_mutex);
-
 	gem_obj = drm_gem_object_lookup(drm, file_priv, handle);
 	if (!gem_obj) {
 		dev_err(drm->dev, "failed to lookup GEM object\n");
-		mutex_unlock(&drm->struct_mutex);
 		return -EINVAL;
 	}
 
 	*offset = drm_vma_node_offset_addr(&gem_obj->vma_node);
 
-	drm_gem_object_unreference(gem_obj);
-
-	mutex_unlock(&drm->struct_mutex);
+	drm_gem_object_unreference_unlocked(gem_obj);
 
 	return 0;
 }