drm: Improve the output_poll_changed description

this callback was used by drm_kms_helper_hotplug_event()

V2: (Thanks for Daniel's suggestions)
- remove the FIXME below.since with the drm_client
- infrastructure and the generic fbdev emulation we've
- resolved this all very neatly now.

V3: Add comments that This hook is deprecated
- new implementation methods instead of this hook

v4: used by drm_kms_helper_hotplug_event()
- drm_fb_helper_hotplug_changed() is not found
instead by drm_kms_helper_hotplug_event()

Signed-off-by: ZhiJie.Zhang <zhangzhijie@loongson.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903032402.11935-1-zhangzhijie@loongson.cn