x86, amd: Don't probe for extended APIC ID if APICs are disabled

If we've logically disabled apics, don't probe the PCI space for the
AMD extended APIC ID.

[ Impact: prevent boot crash under Xen. ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Reported-by: Bastian Blank <bastian@waldi.eu.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>