drm/i915: Fix possible race in intel_dp_add_mst_connector()

drm/i915: Fix possible race in intel_dp_add_mst_connector()

This hasn't caused any issues yet that I'm aware of, but as Ville
Syrjälä pointed out - we need to make sure that
intel_connector->mst_port is set before initializing MST connectors,
since in theory we could potentially check intel_connector->mst_port in
i915_hpd_poll_init_work() after registering the connector but before
having written it's value.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20181106213017.14563-2-lyude@redhat.com
(cherry picked from commit 66a5ab1034be801630816d1fa6cfc30db1a2f0b0)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>