Only demote when needed in the arm64 pmap_change_props_locked

When changing page table properties there is no need to demote a
level 1 or level 2 block if we are changing the entire memory range the
block is mapping. In this case just change the block directly.

Reported by: alc, kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32339