powerpc/mmu: Don't use the cache instructions to zero pages

A page (even physmem) can be marked as cache-inhibited. Attempting to use
'dcbz' to zero a page mapped cache-inhibited triggers an alignment
exception, which is fatal in kernel. This was seen when testing hardware
acceleration with X on POWER9.

At some point in the future, this should be changed to a more straight
forward zero loop instead of bzero(), and a similar change be made to the
other pmaps.

Reported by: pkubaj@