drm/todo: Update backlight todo

Basic helpers have been extracted, now there's a pile more todo still
across the entire tree.

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-57-daniel.vetter@ffwll.ch