[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>