drm/amdgpu: fix user ptr race condition

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: monk liu <monk.liu@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c

@@ -37,6 +37,7 @@ void amdgpu_gem_object_free(struct drm_gem_object *gobj)
 	if (robj) {
 		if (robj->gem_base.import_attach)
 			drm_prime_gem_destroy(&robj->gem_base, robj->tbo.sg);
+		amdgpu_mn_unregister(robj);
 		amdgpu_bo_unref(&robj);
 	}
 }