Support for multiple UI planes can now be easily enabled just by adding
more planes with different index.

For now, add immutable zpos property.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-17-jernej.skrabec@siol.net