drm/i915/mst: Populate connector->ddc

Populate connector->ddc, and thus create the "ddc" symlink
in sysfs for DP MST connectors.

TODO: test that this actually works

References: https://gitlab.freedesktop.org/drm/intel/-/issues/3605


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230829113920.13713-9-ville.syrjala@linux.intel.com


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