radv: add a simple vertex format cache for dynamic vertex bindings
With dynamic vertex bindings the vertex format lookups are a lot more frequent (vs being baked in the pipeline). Add a small lookup cache to avoid the overheads in this path.
Seen in perf traces gputest gimark with zink on radv.
This is a bit of an RFC, this is in on way tuned, but works well to remove the overheads in the perf traces.