drm/i915: Only enable DFP 4:4:4->4:2:0 conversion when outputting YCbCr 4:4:4

Let's not enable the 4:4:4->4:2:0 conversion bit in the DFP unless we're
actually outputting YCbCr 4:4:4. It would appear some protocol
converters blindy consult this bit even when the source is outputting
RGB, resulting in a visual mess.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2914Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111164111.13302-1-ville.syrjala@linux.intel.com
Fixes: 181567aa ("drm/i915: Do YCbCr 444->420 conversion via DP protocol converters")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_dp.c

@@ -3508,8 +3508,8 @@ void intel_dp_configure_protocol_converter(struct intel_dp *intel_dp,
 		drm_dbg_kms(&i915->drm, "Failed to set protocol converter HDMI mode to %s\n",
 			    enableddisabled(intel_dp->has_hdmi_sink));
 
-	tmp = intel_dp->dfp.ycbcr_444_to_420 ?
-		DP_CONVERSION_TO_YCBCR420_ENABLE : 0;
+	tmp = crtc_state->output_format == INTEL_OUTPUT_FORMAT_YCBCR444 &&
+		intel_dp->dfp.ycbcr_444_to_420 ? DP_CONVERSION_TO_YCBCR420_ENABLE : 0;
 
 	if (drm_dp_dpcd_writeb(&intel_dp->aux,
 			       DP_PROTOCOL_CONVERTER_CONTROL_1, tmp) != 1)