drm/connector: hdmi: Attempt YUV422 output if RGB isn't supported

Drop from RGB to YUV422 output if RGB couldn't be supported
within the defined max_bpc and TMDS rates, and then try
dropping max_bpc.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

Author: 6by9

drivers/gpu/drm/display/drm_hdmi_state_helper.c

@@ -653,6 +653,11 @@ hdmi_compute_config(const struct drm_connector *connector,
 
 	ret = hdmi_compute_format_bpc(connector, conn_state, mode, max_bpc,
 				      HDMI_COLORSPACE_RGB);
+
+	if (ret)
+		ret = hdmi_compute_format_bpc(connector, conn_state, mode,
+					      max_bpc, HDMI_COLORSPACE_YUV422);
+
 	if (ret) {
 		if (connector->ycbcr_420_allowed) {
 			ret = hdmi_compute_format_bpc(connector, conn_state,