drm/amd/display: add DalEnableHDMI20 key support

[why]
"DalEnableHDMI20" set to 0, disallow HDMI YCbCr420 and  pixel clock > 340Mhz
Default is enabled.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>