AMD/IOMMU: without XT, x2APIC needs to be forced into physical mode

AMD/IOMMU: without XT, x2APIC needs to be forced into physical mode

The wider cluster mode APIC IDs aren't generally representable. Convert
the iommu_intremap variable into a tristate, allowing the AMD IOMMU
driver to signal this special restriction to the apic_x2apic_probe().
(Note: assignments to the variable get adjusted, while existing
consumers - all assuming a boolean property - are left alone.)

While we are not aware of any hardware/firmware with this as a
restriction, it is a situation which could be created on fully x2apic-
capable systems via firmware settings.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>