powerpc/pseries: iommu cleanup

No need to initialize per-cpu pointer to NULL, it is the default.

Direct dma ops and no setup are the defaults, no need to set for
iommu-off.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>