amd64: restore semantic for VIRT_IN_DMAP()

The macro should check if the virtual address falls into working direct
map, not that it falls into KVA potentially belonging to DMAP. This was
its previous meaning, as can be seen in the only use by DMAP_TO_PHYS().

Fixes: 4dd828c80828637452a8a4e07a64e294c82e5d8b
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D51334