iommu_gas: consolidate find_space helpers

Merge lowermatch and uppermatch into find_space. Eliminate uppermatch
recursion. Merge match_insert into match_one and eliminate some
redundant calculation. Move some initialization out of find_space and
into map (and out from under a lock).

Reviewed by: kib (previous version), alc
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35440