drm/nouveau/mmu: fix comptag memory leak

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/core/memory.c

@@ -91,8 +91,8 @@ nvkm_memory_tags_get(struct nvkm_memory *memory, struct nvkm_device *device,
 	}
 
 	refcount_set(&tags->refcount, 1);
+	*ptags = memory->tags = tags;
 	mutex_unlock(&fb->subdev.mutex);
-	*ptags = tags;
 	return 0;
 }