drm/amd/display: Clear dc_sink after it gets released

[Why]
The dc_sink was released but the pointer on the aconnector was
not cleared.

[How]
Clear it.
Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

@@ -173,8 +173,10 @@ static int dm_dp_mst_get_modes(struct drm_connector *connector)
 		aconnector->edid = edid;
 	}
 
-	if (aconnector->dc_sink && aconnector->dc_sink->sink_signal == SIGNAL_TYPE_VIRTUAL)
+	if (aconnector->dc_sink && aconnector->dc_sink->sink_signal == SIGNAL_TYPE_VIRTUAL) {
 		dc_sink_release(aconnector->dc_sink);
+		aconnector->dc_sink = NULL;
+	}
 
 	if (!aconnector->dc_sink) {
 		struct dc_sink *dc_sink;