iommu/amd: KVM: SVM: Use pi_desc_addr to derive ga_root_ptr

iommu/amd: KVM: SVM: Use pi_desc_addr to derive ga_root_ptr

Use vcpu_data.pi_desc_addr instead of amd_iommu_pi_data.base to get the
GA root pointer.  KVM is the only source of amd_iommu_pi_data.base, and
KVM's one and only path for writing amd_iommu_pi_data.base computes the
exact same value for vcpu_data.pi_desc_addr and amd_iommu_pi_data.base,
and fills amd_iommu_pi_data.base if and only if vcpu_data.pi_desc_addr is
valid, i.e. amd_iommu_pi_data.base is fully redundant.

Cc: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Link: https://lore.kernel.org/r/20250611224604.313496-23-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>