drm/i915/dp_mst: Set connector DSC capabilities and decompression AUX

Similarly to eDP and SST-DP connectors read out the DSC capabilities for
MST connectors as well. Atm these will match the root port's DSC caps
and only used after a follow-up change enables the decompression for
each stream separately (vs. the current way of enabling it only globally
in the first branch device downstream of the root port).

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/20231010112504.2156789-3-imre.deak@intel.com