drm: Add TODO item for requesting memory regions

Add a TODO item about requesting  memory regions for each driver. The
current DRM drivers don't do this consistently.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220125091222.21457-6-tzimmermann@suse.de