powerpc/powernv/npu: Clean up compound table group initialisation

Re-work the control flow a bit so what's going on is a little clearer.
This also ensures the table_group is only initialised once in the P9
case. This shouldn't be a functional change since all the GPU PCI
devices should have the same table_group configuration, but it does
look strange.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200406030745.24595-2-oohall@gmail.com