powerpc/pseries/iommu: Use the iommu table[0] for IOV VF's DDW

This patch basically brings consistency with PowerNV approach
to use the first freely available iommu table when the default
window is removed.

The pSeries iommu code convention has been that the table[0] is
for the default 32 bit DMA window and the table[1] is for the
64 bit DDW.

With VFs having only 1 DMA window, the default has to be removed
for creating the larger DMA window. The existing code uses the
table[1] for that, while marking the table[0] as NULL. This is
fine as long as the host driver itself uses the device.

For the VFIO user, on pSeries there is no way to skip table[0]
as the VFIO subdriver uses the first freely available table.
The window 0, when created as 64-bit DDW in that context would
still be on table[0], as the maximum number of windows is 1.

This won't have any impact for the host driver as the table is
fetched from the device's iommu_table_base.

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
[mpe: Rebase and resolve conflicts]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/171923272328.1397.1817843961216868850.stgit@linux.ibm.com