drm/i915/mst: handle mst pbn_div in intel_dp_mtp_tu_compute_config()

Move mst_state->pbn_div calculation to intel_dp_mtp_tu_compute_config()
to allow further refactoring.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0cc1b507601c9964ebae7d50b1f90b1ce00acb11.1738161945.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>