drm/i915/dp: Account with DSC BW overhead for compressed DP-SST stream BW

drm/i915/dp: Account with DSC BW overhead for compressed DP-SST stream BW

A DSC compressed stream requires FEC (except for eDP), which has a BW
overhead on non-UHBR links that must be accounted for explicitly. Do
that during computing the required BW.

Note that the overhead doesn't need to be accounted for on UHBR links
where FEC is always enabled and so the corresponding overhead is part of
the channel coding efficiency instead (i.e. the overhead is part of the
available vs. the required BW).

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20251215192357.172201-12-imre.deak@intel.com