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

Documentation/gpu/todo.rst

@@ -49,14 +49,18 @@ converted over. Modern compositors like Wayland or Surfaceflinger on Android
 really want an atomic modeset interface, so this is all about the bright
 future.
 
-There is a conversion guide for atomic and all you need is a GPU for a
-non-converted driver (again virtual HW drivers for KVM are still all
-suitable).
+There is a conversion guide for atomic [1]_ and all you need is a GPU for a
+non-converted driver.  The "Atomic mode setting design overview" series [2]_
+[3]_ at LWN.net can also be helpful.
 
 As part of this drivers also need to convert to universal plane (which means
 exposing primary & cursor as proper plane objects). But that's much easier to
 do by directly using the new atomic helper driver callbacks.
 
+  .. [1] https://blog.ffwll.ch/2014/11/atomic-modeset-support-for-kms-drivers.html
+  .. [2] https://lwn.net/Articles/653071/
+  .. [3] https://lwn.net/Articles/653466/
+
 Contact: Daniel Vetter, respective driver maintainers
 
 Level: Advanced