drm/dp_mst: Fix PBN divider calculation for UHBR rates

drm/dp_mst: Fix PBN divider calculation for UHBR rates

The current way of calculating the pbn_div value, the link BW per each
MTP slot, worked only for DP 1.4 link rates. Fix things up for UHBR
rates calculating with the correct channel coding efficiency based on
the link rate.

v2:
- Return the fractional pbn_div value from drm_dp_get_vc_payload_bw().
v3:
- Fix rounding up quotient while calculating req_slots. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231117150929.1767227-1-imre.deak@intel.com