drm/radeon: don't do mst probing if MST isn't enabled.

This causes an oops as we haven't initialised the mst
layer.
Reported-by: Dave Jones <&lt;davej@codemonkey.org.uk>
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

@@ -666,6 +666,9 @@ radeon_dp_mst_probe(struct radeon_connector *radeon_connector)
 	int ret;
 	u8 msg[1];
 
+	if (!radeon_mst)
+		return 0;
+
 	if (dig_connector->dpcd[DP_DPCD_REV] < 0x12)
 		return 0;