x86_64: don't need set default res if only have one root bus

if there's only one root bus there's no need to split resources.

This patch fixes the issue described at:

  http://lkml.org/lkml/2008/4/10/304

Reported-and-bisected-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>