drm/radeon: fix user ptr race condition

Port of amdgpu patch 9298e52f8b51d1e4acd68f502832f3a97f8cf892.

Signed-off-by: Christian König <christian.koenig@amd.com>
CC: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/radeon/radeon_gem.c b/drivers/gpu/drm/radeon/radeon_gem.c
index 013ec71..3dcc573 100644
--- a/drivers/gpu/drm/radeon/radeon_gem.c
+++ b/drivers/gpu/drm/radeon/radeon_gem.c
@@ -36,6 +36,7 @@
 	if (robj) {
 		if (robj->gem_base.import_attach)
 			drm_prime_gem_destroy(&robj->gem_base, robj->tbo.sg);
+		radeon_mn_unregister(robj);
 		radeon_bo_unref(&robj);
 	}
 }