drm/i915: Fix the MST PBN divider calculation

drm/i915: Fix the MST PBN divider calculation

Atm the driver will calculate a wrong MST timeslots/MTP (aka time unit)
value for MST streams if the link parameters (link rate or lane count)
are limited in a way independent of the sink capabilities (reported by
DPCD).

One example of such a limitation is when a MUX between the sink and
source connects only a limited number of lanes to the display and
connects the rest of the lanes to other peripherals (USB).

Another issue is that atm MST core calculates the divider based on the
backwards compatible DPCD (at address 0x0000) vs. the extended
capability info (at address 0x2200). This can result in leaving some
part of the MST BW unused (For instance in case of the WD19TB dock).

Fix the above two issues by calculating the PBN divider value based on
the rate and lane count link parameters that the driver uses for all
other computation.

Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/2977
Cc: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjala <ville.syrjala@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjala <ville.syrjala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125173636.1733812-2-imre.deak@intel.com