drm/dp_mst: Manually overwrite PBN divider for calculating timeslots

[why]
For DSC case we cannot use topology manager's PBN divider
variable. The default divider does not take FEC into account.
Therefore the driver has to calculate its own divider based
on the link rate and lane count its handling, as it is hw specific.

[how]
Pass pbn_div as an argument, which is used if its more than
zero, otherwise default topology manager's pbn_div will be used.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>