drm/i915: Extract intel_dp_mst_add_properties()

Move the DP MST connector property setup into its own function
so it's not spread all over intel_dp_add_mst_connector().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220912111814.17466-9-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>