pmap_demote_{l2,pde}: never invalidate wired mappings

Rather than demoting a superpage mapping that hasn't been accessed, we
ordinarily invalidate it. However, if the mapping is wired,
invalidation is problematic because the page fault to reinstantiate the
mapping could sleep. Instead, demote the wired mapping with the
accessed flag cleared in the PTEs.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51431