amdiommu_create_dev_tbl(): eliminate magic '3'

[kib]

Summary

in the calculation of the high dev table segment register index.
Use fixed array to translate index into the register number.

[jah]

Oops, I didn't see this until after I wrote and tested my own version: https://reviews.freebsd.org/D47769

[kib]

In D47768#1089266, @jah wrote:

Oops, I didn't see this until after I wrote and tested my own version: https://reviews.freebsd.org/D47769

Commit yours please, the actual fix is from you.