drm/amd/display: Fix bpc calculation for specific encodings

[Why]
1. YCbCr 4:2:2 8bpc/10bpc modes are blocked for HDMI by policy
2. A YCbCr 4:2:0 calculation error blocked some 4:2:0 timing modes

[How]
YCbCr 4:2:2 8bpc/10bpc modes are allowed for HDMI
Fix YCbCr 4:2:0 calculation error

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Bing Guo <Bing.Guo@amd.com>
Reviewed-by: Chris Park <chris.park@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>