drm/docs: Add todo entry for drm_fb_helper_fbdev_setup()

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/20171215175119.36181-4-noralf@tronnes.org