drm/todo: Add item about modeset properties

Add some information about pre-atomic modeset properties alongside a
list of suggestions how to handle the different instances.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603170434.2363446-1-emil.l.velikov@gmail.com