drm/fb-helper: Add .last_close and .output_poll_changed helpers

This adds helpers for the drm_driver->last_close and the
drm_mode_config_funcs->output_poll_changed callbacks.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171030153951.56269-4-noralf@tronnes.org