amdiommu: use static array for device table base register offsets

This is a somewhat cleaner and more future-proof way to get the correct
device table offsets.

Reviewed by: kib
Reported by: crest_freebsd_rlwinm.de
Fixes: 5035db22 "amdiommu: Fix device table segment

			base register offsets"

Differential Revision: https://reviews.freebsd.org/D47769