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

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