mips busdma: bzero map on alloc

Maps from the mips busdma dmamap_zone were not completely initialized.
In particular, pagesneeded and pagesreserved were not initialized. This
could cause a crash.

Remove some dead fields from mips struct bus_dmamap while here.

Reported by: brooks
Reviewed by: ian
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22638