drm/fb-helper: Clarify use of last_close and output_poll_changed

Clarify documentation in the use of struct drm_driver.last_close and
struct drm_mode_config_funcs.output_poll_changed. Those callbacks should
not be said for fbdev implementations on top of struct drm_client_funcs.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-24-tzimmermann@suse.de