KVM: SVM: Update "APICv in x2APIC without x2AVIC" in avic.c, not svm.c

KVM: SVM: Update "APICv in x2APIC without x2AVIC" in avic.c, not svm.c

Set the "allow_apicv_in_x2apic_without_x2apic_virtualization" flag as part
of avic_hardware_setup() instead of handling in svm_hardware_setup(), and
make x2avic_enabled local to avic.c (setting the flag was the only use in
svm.c).

Tag avic_hardware_setup() with __init as necessary (it should have been
tagged __init long ago).

No functional change intended (aside from the side effects of tagging
avic_hardware_setup() with __init).

Acked-by: Naveen N Rao (AMD) <naveen@kernel.org>
Tested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20250919215934.1590410-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>