amd64 pmap: More unification for psind = 1 vs 2 in pmap_enter_largepage().

Move

pkru check
wait for page alloc
wire accounting update
asserting allowed updates for valid mappings

out of psind conditions.

Also add assert that psind references supported page size.
Remove not true comment.
Avoid uneccessary page table walks from top level.

Reviewed by: alc, markj (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26513