drm/radeon: fix sparse warning: Should it be static?

Impact: make symbol static.

Fix this sparse warning:
  drivers/gpu/drm/radeon/radeon_dp_mst.c:332:38: warning: symbol 'mst_cbs' was not declared. Should it be static?
Signed-off-by: Dmitry Rozhkov <dmitry.rozhkov@linux.intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_dp_mst.c

@@ -328,7 +328,7 @@ static void radeon_dp_mst_hotplug(struct drm_dp_mst_topology_mgr *mgr)
 	drm_kms_helper_hotplug_event(dev);
 }
 
-const struct drm_dp_mst_topology_cbs mst_cbs = {
+static const struct drm_dp_mst_topology_cbs mst_cbs = {
 	.add_connector = radeon_dp_add_mst_connector,
 	.register_connector = radeon_dp_register_mst_connector,
 	.destroy_connector = radeon_dp_destroy_mst_connector,