drm/todo: Add atomic modesetting references

The section about converting existing KMS drivers to atomic modesetting
mentions the existence of a conversion guide, but does not reference it.
While the guide is old and rusty, it still contains useful information,
so add a link to it.  Also link to the LWN.net articles that give an
overview about the atomic mode setting design.

While at it, remove the reference to unconverted virtual HW drivers, as
they've been converted.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6809d0fda0716892cbccf0ee272481032251026d.1689779916.git.geert+renesas@glider.be