drm/radeon: use helper for mst connector dpms.

I noticed my monitor didn't power off when it should,
this should fix it.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_dp_mst.c

@@ -257,14 +257,8 @@ radeon_dp_mst_connector_destroy(struct drm_connector *connector)
 	kfree(radeon_connector);
 }
 
-static int radeon_connector_dpms(struct drm_connector *connector, int mode)
-{
-	DRM_DEBUG_KMS("\n");
-	return 0;
-}
-
 static const struct drm_connector_funcs radeon_dp_mst_connector_funcs = {
-	.dpms = radeon_connector_dpms,
+	.dpms = drm_helper_connector_dpms,
 	.detect = radeon_dp_mst_detect,
 	.fill_modes = drm_helper_probe_single_connector_modes,
 	.destroy = radeon_dp_mst_connector_destroy,