vm: remove pa_index from the machine-independent layer

After the demise of vm_page_lock(), the only remaining uses of
pa_index() are in various pmap implementations. In many cases, e.g.,
amd64, the pmap implementations already provided their own definitions,
often identical to the machine-independent one. For those that didn't
provide one, this change adds it.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D50823