busdma: tidy bus_dma_run_filter() functions

[mhorne]

Summary

After removing filter functionality, the naming doesn't clearly
represent what the function does, so try to address this. Include some
code clarity and style improvements.

Create a common version in subr_busdma_bounce.c, used by most
implementations. powerpc still needs its own version of the function,
due to its dmat->iommu == NULL check.

No functional change intended.

[mhorne]

sys/arm64/arm64/busdma_bounce.c

228–234

Here it seems like the logic is changed, but it is not. In practice BF_COULD_BOUNCE is set whenever the tag's alignment requirement is > 1.