drm/i915/dp_mst: Enable DSC passthrough

Enable passing through DSC streams to the sink in last branch devices.

v2:
- Fix the DPCD register address while setting/clearing the passthrough
  flag.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
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/20231030155843.2251023-25-imre.deak@intel.com