busdma: tidy bus_dma_run_filter() functions

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.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42896