drm/stm: ltdc: add support of the dynamic z-order

Zpos property is immutable for all hardware versions except the last
version (0x40100) which support the blending order feature
(dynamic z-order).

Signed-off-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/20220603134459.593379-1-yannick.fertre@foss.st.com