backend-drm: Hard-code zpos values if HW doesn't exposes them

backend-drm: Hard-code zpos values if HW doesn't exposes them

This is based on the assumption that overlays are in between cursor and
primary plane and it is required to be able to assign views to planes,
even if the driver doesn't not expose such property.

As we hard-code them as immutable the commit part would not need any
further modifications.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Daniel Stone <daniel.stone@collabora.com>