drm/amd/display: block DP YCbCr420 modes

[why]
Currently not supported, will black screen when set.

[How]
Fail validate timing helper for those modes.
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce/dce_link_encoder.c

@@ -662,6 +662,9 @@ bool dce110_link_encoder_validate_dp_output(
 	const struct dce110_link_encoder *enc110,
 	const struct dc_crtc_timing *crtc_timing)
 {
+	if (crtc_timing->pixel_encoding == PIXEL_ENCODING_YCBCR420)
+		return false;
+
 	/* default RGB only */
 	if (crtc_timing->pixel_encoding == PIXEL_ENCODING_RGB)
 		return true;

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_link_encoder.c

@@ -606,6 +606,9 @@ bool dcn10_link_encoder_validate_dp_output(
 	const struct dcn10_link_encoder *enc10,
 	const struct dc_crtc_timing *crtc_timing)
 {
+	if (crtc_timing->pixel_encoding == PIXEL_ENCODING_YCBCR420)
+		return false;
+
 	/* default RGB only */
 	if (crtc_timing->pixel_encoding == PIXEL_ENCODING_RGB)
 		return true;