busdma_iommu: simplify split logic

iommu_bus_dmamap_load_something1 includes code for handling the
possibility of splitting a buffer that is needlessly complex.
Simplify it.

Reviewed by: alc, kib
MFC after: 3 weeks
Tested by: pho (previous revisions)
Differential Revision: https://reviews.freebsd.org/D35232

(cherry picked from commit 04e86ae357b21fce238a90f31198b25975d0f479)