drm/meson: Add zpos immutable property to planes

Add immutable zpos property to primary and overlay planes to specify
the current fixed zpos position.

Fixes: f9a23481

 ("drm/meson: Support Overlay plane for video rendering")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190429075247.7946-1-narmstrong@baylibre.com