drm/sun4i: Add multi plane support to DE2 driver

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