drm/stm: ltdc: attach immutable zpos property to planes

Defines plane ordering by hard-coding an immutable Z position from the
first plane, used as primary layer, to the next ones as overlay in order
of instantiation.

This zpos is only an information as it is not possible to modify it,
blending operations are still applied from the top to the bottom layer.

This patch helps to remove a warning message from the Android
Hardware Composer.

Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Acked-by: Yannick Fertre <yannick.fertre@foss.st.com>
Reviewed-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210907151323.5850-1-raphael.gallais-pou@foss.st.com