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

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

An earlier change with the same title (commit 1ba66a870eba) altered only
the path where x2apic_phys was already set to false (perhaps from the
command line). The same of course needs applying when the variable
wasn't modified yet from its initial value.

Reported-by: Elliott Mitchell <ehem+xen@m5p.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 0d2686f6b66b4b1b3c72c3525083b0ce02830054
master date: 2023-03-21 09:23:25 +0100