drm/docs: Record TODO about plane clipping

It's such a mess that it's become a FAQ :(

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170226193851.3245-1-daniel.vetter@ffwll.ch

Documentation/gpu/todo.rst

@@ -63,6 +63,18 @@ do by directly using the new atomic helper driver callbacks.
 
 Contact: Daniel Vetter, respective driver maintainers
 
+Clean up the clipped coordination confusion around planes
+---------------------------------------------------------
+
+We have a helper to get this right with drm_plane_helper_check_update(), but
+it's not consistently used. This should be fixed, preferrably in the atomic
+helpers (and drivers then moved over to clipped coordinates). Probably the
+helper should also be moved from drm_plane_helper.c to the atomic helpers, to
+avoid confusion - the other helpers in that file are all deprecated legacy
+helpers.
+
+Contact: Ville Syrjälä, Daniel Vetter, driver maintainers
+
 Convert early atomic drivers to async commit helpers
 ----------------------------------------------------