anv: Use resolve_device_entrypoint for dispatch init

There's no good reason to have the "which table do I use?" code duplicated twice. The only advantage to the way we were doing it before was that we could move the switch statement outside the loop. If this is ever an actual device initialization perf problem that someone cares about, we can optimize that when the time comes. For now, the duplicated cases are simply a platform-enabling pit-fall.