drm/sun4i: backend: Set a default zpos in our reset hook

The plane state zpos value will be set only if there's an existing state
attached to the plane when creating the property.

However, this is not the case during the probe, and we therefore need to
put our default value in our reset hook.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b6a183234f0ad5a9a58c780c9cabbe29cbf40888.1516617243.git-series.maxime.ripard@free-electrons.com