backend-drm: Assign the primary plane the lowest zpos value

backend-drm: Assign the primary plane the lowest zpos value
when switching to mixed-mode of compositing

This way we avoid an (incorrect) duplicate check of zpos values. Also,
this would be needed because the renderer needs have the lowest zpos value
available as we don't (yet) properly support underlays, the primary
plane serves as our renderer.

Adds also a check to see if we try to assign a view to a plane with
a lower zpos value than the one assigned to the primary when switching
to mixed-mode of compositing.

Fixes: #304
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>