drm/i915/dp: Specify the FEC overhead as an increment vs. a remainder

A follow-up patch will add up all the overheads on a DP link, where it
makes more sense to specify each overhead factor in terms of the added
overhead amount vs. the reciprocal remainder (of usable BW remaining
after deducting the overhead). Prepare for that here, keeping the
existing behavior.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-12-imre.deak@intel.com