drm/sun4i: backend: Make zpos configurable

Now that we have everything in place, we can make zpos configurable now.
Change the zpos property from an immutable one to a regular.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0ab187956855db86972d936e6751181649e0d035.1518802627.git-series.maxime.ripard@bootlin.com