drm/amd/display: Correct slice width calculation for YCbCr420

drm/amd/display: Correct slice width calculation for YCbCr420

[Why]
-OVT compliance testing for 5120x2880p300Hz YCbCr420 was failing due to
incorrect slice width being calculated

[How]
-Ensure slice width is divisible by 2 for 420 to comply with spec

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Relja Vojvodic <rvojvodi@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>