drm/amd/display: Add link encoder dp_ycbcr420_supported feature flag

[Why]
Need separate feature flag for DP 4:2:0 support, since existing
flag is used for HDMI

[How]
Added dp_ycbcr420_supported to struct encoder_feature_support

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>