powerpc64/pmap: micro-optimize some PVO-PTE logic

Summary:
moea64_pte_sync_native() and moea64_pte_unset_native() don't need the
full PTE created, they only need to check that the PVO has a matching
PTE to the PTE in the page table. Don't waste time creating the full
PTE in this case.

Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D22341