amd64 pmap_allocpte_nosleep(): stop testing tautological condition

Enabled PTI for given pmap is equivalent to pm_ucr3 being valid is
equivalent to root userspace page table page pm_pmltopu being
allocated.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D45966