drm/amd/display: Drop to fail-safe mode if edid is bad

Provide the connector with a single fail-safe mode of 640x480 for CTS
tests instead of providing a list of possible base modes.
Signed-off-by: NMikita Lipski <mikita.lipski@amd.com>
Reviewed-by: NTony Cheng <Tony.Cheng@amd.com>
Acked-by: NHarry Wentland <harry.wentland@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -3424,6 +3424,10 @@ static int amdgpu_dm_connector_get_modes(struct drm_connector *connector)
 	struct edid *edid = amdgpu_dm_connector->edid;
 
 	encoder = helper->best_encoder(connector);
+
+	if (!edid || !drm_edid_is_valid(edid))
+		return drm_add_modes_noedid(connector, 640, 480);
+
 	amdgpu_dm_connector_ddc_get_modes(connector, edid);
 	amdgpu_dm_connector_add_common_modes(encoder, connector);