drm/todo: Add entry about dealing with brightness control on devices with > 1 panel

Add an entry summarizing the discussion about dealing with brightness
control on devices with more then 1 internal panel.

The original discussion can be found here:
https://lore.kernel.org/dri-devel/20220517152331.16217-1-hdegoede@redhat.com/



Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>