drm: update todo.rst

- debugfs cleanup has moved forward thanks to the cleanup Wambui has
  done

Acked-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Wambui Karuga <wambui.karugax@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200929150333.3441576-1-daniel.vetter@ffwll.ch