drm/probe-helper: add drm_kms_helper_connector_hotplug_event

This function is the same as drm_kms_helper_hotplug_event, but takes
a connector instead of a device.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018084707.32253-3-contact@emersion.fr