drm: Remove struct drm_mode_config_funcs.output_poll_changed

The output_poll_changed hook in struct drm_mode_config_funcs is
unused. Remove it. The helper drm_client_dev_hotplug() implements
the callback's functionality.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812083000.337744-10-tzimmermann@suse.de