drm/amd/display: Set up pixel encoding for YCBCR422

drm/amd/display: Set up pixel encoding for YCBCR422

[Why]
fill_stream_properties_from_drm_display_mode() will not configure pixel
encoding to YCBCR422 when the DRM color format supports YCBCR422 but not
YCBCR420 or YCBCR4444.  Instead it will fallback to RGB.

[How]
Add support for YCBCR422 in pixel encoding mapping.

Suggested-by: Mauri Carvalho <mcarvalho3@lenovo.com>
Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Mario Limonciello <Mario.Limonciello@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>