Tigthen up kern_prefetch.h includes. The DMAP check was not committed in
here with HPTS so these includes aren't necessary.

Comments from rrs:
Yeah I had those in there in our version so I could
validate the range in amd64 with the DMAP_VM but for
some reason those don't work in head anymore.. Forgot
to purge the headers :)

Submitted by: Kevin Bowling
Reviewed by: rrs
Differential Revision: https://reviews.freebsd.org/D15164