drm/nouveau/disp: use drm_kms_helper_connector_hotplug_event()

This adds more information to the hotplug uevent and lets user-space
know that it's about a particular connector only.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230620181547.272476-1-contact@emersion.fr