drm/i915/dp: Account for channel coding efficiency on UHBR links

Apply the correct BW allocation overhead and channel coding efficiency
on UHBR link rates, similarly to DP1.4 link rates.

Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116131841.1588781-6-imre.deak@intel.com