Avoid spurious EINVAL in amd64 pmap_change_attr().

Do not try to change attributes for DMAP when working on a mapping
which is not covered by the DMAP. This was reported on real system
where a BAR of a device (NTB) was mapped outside the PCI window.

Reported and tested by: mav
Reviewed by: jhb, mav
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D6668