powerpc/iommu/ioda2: Enable compile with IOV=on and IOMMU_API=off

powerpc/iommu/ioda2: Enable compile with IOV=on and IOMMU_API=off

The pnv_pci_ioda2_unset_window() function is used to do the final
cleanup of a DMA window being released:
- via VFIO ioctl by the guest request;
- via unplugging a virtual PCI function.
However the function was under #ifdef CONFIG_IOMMU_API and was missing.

This moves the helper outside of IOMMU_API block and enables it
for either or both IOMMU_API and PCI_IOV.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>