iommu/amd: Re-factor guest virtual APIC (de-)activation code

Re-factore the logic for activate/deactivate guest virtual APIC mode (GAM)
into helper functions, and export them for other drivers (e.g. SVM).
to support run-time activate/deactivate of SVM AVIC.

Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>