[PowerPC] fix panic reading /dev/kmem on !DMAP machines

This fixes /dev/kmem causing panic on machines not using DMAP.

Found when running libkvm Kyua test case on QEMU VM with no
Huge Pages support.

Reviewed by: jhibbits, luporl
Approved by: jhibbits (mentor)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D23776