i915/display/dp: send a more fine-grained link-status uevent

When link-status changes, send a hotplug uevent which contains the
connector ID. That way, user-space can more easily figure out that
only this connector has been updated.

Changes in v4: avoid sending two uevents (Ville)

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018084707.32253-7-contact@emersion.fr