anv: Use resolve_device_entrypoint for dispatch init

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.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5530>