drm/todo: Add entry for fb funcs related cleanups

We're doing a great job for really simple drivers right now, but still
a lot of boilerplate for the bigger ones.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127180035.416209-3-daniel.vetter@ffwll.ch