i915 can enable aux device nodes for DP MST by calling
drm_dp_mst_connector_late_register()/
drm_dp_mst_connector_early_unregister(),
so let's hook that up.

Changes since v1:
* Call intel_connector_register/unregister() from
  intel_dp_mst_connector_late_register/unregister() so we don't lose
  error injection - Ville Syrjälä
Changes since v2:
* Don't forget to clean up if intel_connector_register() fails - Ville

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: "Lee, Shawn C" <shawn.c.lee@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310195122.1590925-1-lyude@redhat.com