amdgpu: use drm_kms_helper_connector_hotplug_event

When updating a single connector, use
drm_kms_helper_connector_hotplug_event instead of
drm_kms_helper_hotplug_event.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018084707.32253-5-contact@emersion.fr