drm/amd/display: Switch the custom "max bpc" property to the DRM prop

[Why]
The custom "max bpc" property was added to limit color depth while the
DRM one was still being merged. It's been a few kernel versions since
then and this TODO was still sticking around.

[How]
Attach the DRM max bpc property to the connector and drop all of our
custom property management. Set the max bpc to 8 by default since
DRM defaults to the max in the range which would be 16 in this case.

No behavioral changes are intended with this patch, it should just be
a refactor.

v2: Don't force 8bpc when no state is given

Cc: Leo Li <sunpeng.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>