drm/amd/display: Fix incorrect deep color setting in YCBCR420 modes

[Why]
HDMI 2.0 HF-VSDB in EDID defines supported color depths in YCBCR420 modes.
But we did not honor these bit masks when choosing pixel encoding.
HDMI 2.0 compliance tests with deep color and YCBCR420 failed as a result.

[How]
Cap color depth based on y420_dc_modes from EDID.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>