drm/docs: Add todo entry for simple modeset suspend/resume

Add entry for conversion of drivers to new helpers.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106191812.38927-7-noralf@tronnes.org