drm/i915/dp: Factor out helpers to compute the link limits

Factor out helpers that DP / DP_MST encoders can use to compute the link
rate/lane count and bpp limits. A follow-up patch will call these to
recalculate the limits if DSC compression is required.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921195159.2646027-2-imre.deak@intel.com