iommu_gas: don't remove and insert boundary entry

Avoid removing an item in iommu_gas_free_region only to reinsert it,
by avoiding removing an entry that is either first_place or
last_place.

Reviewed by: kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D36597

(cherry picked from commit 87cd087a4ae7b272986ce3bcf27ffe4930f15e55)