drm/i915/dp_mst: Use connector DSC DPCD in intel_dp_mst_mode_valid_ctx()

Use the connector's DSC DPCD capabilities in
intel_dp_mst_mode_valid_ctx().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231006133727.1822579-19-imre.deak@intel.com