drm/i915/dp: Add intel_dp_mode_valid_with_dsc()

drm/i915/dp: Add intel_dp_mode_valid_with_dsc()

Add intel_dp_mode_valid_with_dsc() and call this for an SST/MST mode
validation to prepare for a follow-up change using a way to verify the
mode's required BW the same way this is done elsewhere during state
computation (which in turn depends on the mode's effective data rate
with the corresponding BW overhead).

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20251222153547.713360-13-imre.deak@intel.com