drm/i915/dp: Add a separate function to reduce the link parameters

A follow-up patch will add an alternative way to reduce the link
parameters in BW order on MST links, prepare for that here.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722165503.2084999-8-imre.deak@intel.com