drm/i915/dp: Limit max link bpp properly to a fractional value on SST

drm/i915/dp: Limit max link bpp properly to a fractional value on SST

The maximum link bpp - determined by the link BW for instance - can be
fractional, handle this properly during computing the link bpp on SST.

This keeps the pipe joiner specific maximum link bpp as a rounded-down
integer value still, changing that to a fractional value is left for
later.

v2: Align the min/max bpp value to the bpp step.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-8-imre.deak@intel.com