Basic brute-force plane allocation

[emersion]

The first allocation scheme to get working prior to incremental updates is a basic brute-force plane allocation. The idea is to try all layer → plane combinations.

The tricky part is that some drivers require the primary plane to be allocated first. An atomic test commit with the primary plane disabled but overlay planes enabled will fail.

Thus we really need to find an FB for the primary plane first. Let's do the allocation the other way around: plane → layer. That way we can choose to allocate the primary plane first.