drm/i915/dp_mst: Account with the DSC DPT bpp limit on MTL

The DPT/DSC bpp limit should be accounted for on MTL platforms as well,
do so.

Bspec: 49259

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-6-imre.deak@intel.com