amd64 efirt: do not flush cache for runtime pages

We actually do not know is it safe or not to flush cache for random
BAR/register page existing in the system. It is well-known that for
instance LAPICs cannot tolerate cache flush. As report indicates,
there are more such devices.

This issue typically affects AMD machines which do not report self-snoop,
causing real CLFLUSH invocation on the mapped pages. Intels do self-snoop,
so this change should be nop for them, and unsafe devices, if any, are
already ignored.

Reported and tested by: manu
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32318