pmap: Degrade pmap_page_set_attr*() into a no-op on same attribute

For 32-bit arm, move the no-op test that was already in place at start
of the function so that it stays first even if the '#if 0' block around
the call to sf_buf_invalidate_cache() is uncommented at some point (if
ever).

Reviewed by: jeffpc_josefsipek.net, kib
MFC after: 10 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51253