drm/sun4i: Add TODO comment about supporting scaling with the backend

The backend allows integer-only scaling but can handle alpha components,
unlike the frontend. It could be useful to add support for this
eventually, so add a short TODO comment describing the situation.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-4-paul.kocialkowski@bootlin.com