drm/bridge_connector: 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: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230620175506.263109-1-contact@emersion.fr