bus_dma: Deduplicate locking helper functions.

• Move busdma_lock_mutex to subr_bus_dma.c.

• Move _busdma_lock_dflt to subr_bus_dma.c. This function was named a couple of different things previously. It is not a public API but an internal helper used in place of a NULL pointer. The prototype is in <sys/bus_dma.h> as not all backends include <sys/bus_dma_internal.h>.

Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D33694