drm/amd/display: Add 6bpc RGB case for dcn32 output bpp calculations

[Why]
Current DCN32 calculation doesn't consider RGB 6bpc for the DP case.
This results in an invalid output bpp being calculated when DSC is not
enabled in the configuration, failing the mode validation.

[How]
Add special case to handle 6bpc RGB in the output bpp calculation.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>