drm/i915/dp_mst: Enable fractional link bpps on MST if the bpp is forced

drm/i915/dp_mst: Enable fractional link bpps on MST if the bpp is forced

Enable using a fractional (compressed) link bpp on MST links, if this is
supported and the link bpp is forced. Fractional link bpps will be
enabled by default as a follow-up change after testing this
functionality within a set of commonly used MST monitors and docks/hubs
which support it.

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-13-imre.deak@intel.com