x86: NMI watchdog: Downgrade helper

A downgrade helper for the NMI watchdog to be used in all places where
the I/O APIC watchdog may have been requested, but the I/O APIC is found
not to be there or meant to be left disabled.  This is so that the
reconfiguration is cosistent and defined in a single place only.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>