drm/amd/display: Fix ineffective setting of max bpc property

[Why]
Regression was introduced where setting max bpc property has no effect
on the atomic check and final commit. It has the same effect as max bpc
being stuck at 8.

[How]
Correctly propagate max bpc with the new connector state.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org