drm/todo: Add item for the plane->atomic_check confusion

It's frankly a mess, and the confusion around plane_state->crtc/fb
that I fixed up in this series is the least of the problems. Add a
todo as a future note of how this could be done a lot better, and with
a lot less driver confusion.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-1-daniel.vetter@ffwll.ch