anv: Allow enumerating multiple physical devices

anv: Allow enumerating multiple physical devices

Instead of having a single physical device in anv_instance, have a
linked list of them.  What we have now works today because we our GPUs
are build into the CPU and so you're guaranteed to only ever have one of
them.  One day, that will change and we want ANV to be ready.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3461>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3461>