backend-drm: Construct a zpos candidate list of planes

backend-drm: Construct a zpos candidate list of planes

In order to better optimize view assignment to HW planes, we construct
an intermediary zpos candidate list which is used aggregate all suitable
planes for handling scan-out capable client buffers.

We go over it twice: once to construct it and once to pick-and-choose a
suitable plane based its highest zpos position.

In order to maintain the view order correctly we track current zpos
value being applied to the plane state and use it when trying to place
a view into a plane.

Pass the computed zpos value to be applied to the plane state.

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