drm/amd/display: Do not warn NULL dc_sink if forcing connector

[why]
There is a DRM_ERROR when the dc_sink is NULL and
there should not be this warning when the connector
is forced.

[how]
Do not warn if dc_sink is NULL if the connector
is forced.
Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -5833,7 +5833,8 @@ enum drm_mode_status amdgpu_dm_connector_mode_valid(struct drm_connector *connec
 
 	dc_sink = to_amdgpu_dm_connector(connector)->dc_sink;
 
-	if (dc_sink == NULL) {
+	if (dc_sink == NULL && aconnector->base.force != DRM_FORCE_ON_DIGITAL &&
+				aconnector->base.force != DRM_FORCE_ON) {
 		DRM_ERROR("dc_sink is NULL!\n");
 		goto fail;
 	}