drm/sun4i: backend: Add support for zpos

Our various planes have a configurable zpos, that combined with the pipes
allow to configure the composition.

Since the interaction between the pipes, zpos and alphas framebuffers is
not trivial, let's just enable the zpos as an immutable property for now,
and use that zpos in our atomic_update part.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b006853e908bd06661c5bc1f2191121523bce0e4.1516617243.git-series.maxime.ripard@free-electrons.com