powerpc/pseries: unwind dma_get_required_mask_pSeriesLP a bit

Call dma_get_required_mask_pSeriesLP directly instead of dma_iommu_ops
to simply the code a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>