drm/todo: Add link to old debugfs RFC

I always forget where it was, store it until this gets picked up by an
internship again.

Cc: Wambui Karuga <wambui.karugax@gmail.com>
Cc: Melissa Wen <melissa.srw@gmail.com>
Acked-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421152911.1871473-1-daniel.vetter@ffwll.ch