drm/amd/display: Enable DP YCbCr420 mode support for DCN10

[WHY]
In DCN10 when a panel with YCbCr420 capability is connected via
USB-C to HDMI active dongle, no YCbCr420 option is listed in
Radeon settings.

[HOW]
Enable DP YCbCr420 mode support for DCN10

Signed-off-by: Gary Li <gary.li@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>