x86/mm: Swap mfn_valid() to use mfn_t

x86/mm: Swap mfn_valid() to use mfn_t

Replace one opencoded mfn_eq() and some coding style issues on altered lines.
Swap __mfn_valid() to being bool, although it can't be updated to take mfn_t
because of include dependencies.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Julien Grall <julien.grall@arm.com>