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

Documentation/gpu/todo.rst

@@ -546,6 +546,8 @@ There's a bunch of issues with it:
   this (together with the drm_minor->drm_device move) would allow us to remove
   debugfs_init.
 
+Previous RFC that hasn't landed yet: https://lore.kernel.org/dri-devel/20200513114130.28641-2-wambui.karugax@gmail.com/
+
 Contact: Daniel Vetter
 
 Level: Intermediate