drm/amd/display: adding ycbcr420 pixel encoding for hdmi

[why]
HDMI EDID's VSDB contains spectial timings for specifically
YCbCr 4:2:0 colour space. In those cases we need to verify
if the mode provided is one of the special ones has to use
YCbCr 4:2:0 pixel encoding for display info.
[how]
Verify if the mode is using specific ycbcr420 colour space with
the help of DRM helper function and assign the mode to use
ycbcr420 pixel encoding.

Tested-by: Mike Lothian <mike@fireburn.co.uk>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>