drm/amd/display: Add YCbCr420 only support for HDMI 4K@60

[Why]
Some monitors mark 4K@60 capable HDMI port only have 300MHz TMDS
maximum, but the edid includes 4K@60 mode in cea extension block.

[How]
To enable 4K@60, need to limit BW by allowing YCbCr420 ONLY mode.
Add YCbCr420 only support for monitors that do not fully support
HDMI2.0, e.g., ASUS PA328. The YCbCr420 only support applies to
DCN, DCE112 or higher.
Signed-off-by: NJerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: NCharlene Liu <Charlene.Liu@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

@@ -3509,7 +3509,6 @@ void amdgpu_dm_connector_init_helper(struct amdgpu_display_manager *dm,
 	aconnector->base.stereo_allowed = false;
 	aconnector->base.dpms = DRM_MODE_DPMS_OFF;
 	aconnector->hpd.hpd = AMDGPU_HPD_NONE; /* not used */
-
 	mutex_init(&aconnector->hpd_lock);
 
 	/* configure support HPD hot plug connector_>polled default value is 0
@@ -3518,9 +3517,13 @@ void amdgpu_dm_connector_init_helper(struct amdgpu_display_manager *dm,
 	switch (connector_type) {
 	case DRM_MODE_CONNECTOR_HDMIA:
 		aconnector->base.polled = DRM_CONNECTOR_POLL_HPD;
+		aconnector->base.ycbcr_420_allowed =
+			link->link_enc->features.ycbcr420_supported ? true : false;
 		break;
 	case DRM_MODE_CONNECTOR_DisplayPort:
 		aconnector->base.polled = DRM_CONNECTOR_POLL_HPD;
+		aconnector->base.ycbcr_420_allowed =
+			link->link_enc->features.ycbcr420_supported ? true : false;
 		break;
 	case DRM_MODE_CONNECTOR_DVID:
 		aconnector->base.polled = DRM_CONNECTOR_POLL_HPD;