drm/i915/dp_mst: Validate compressed bpp vs. platform restrictions

drm/i915/dp_mst: Validate compressed bpp vs. platform restrictions

Atm TGL supports only a fixed set of valid DSC compressed bpps
(6,8,10,12,15), but this is not taken into account while looking for a
bpp in the minimum..maximum compressed bpp range.

This happened to work only by chance since atm from the above min..max
range it's always the maximum bpp that is selected, which is one of the
above valid bpps (see mst_stream_dsc_compute_link_config() ->
intel_dp_dsc_nearest_valid_bpp()). Before selecting a bpp however, the
bpp's BW requirement should be checked wrt. to the MST total link BW;
after doing that - in a follow-up change - the validity of any bpp in
the min..max range must be ensured before the bpp is selected, do that
here.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-4-imre.deak@intel.com