powerpc/pseries/iommu: Add additional checks when changing iommu mask

Do not check dma supported until we have chosen the right dma ops.
Check that the device is pci before treating it as such.

Check the mask is supported by the selected dma ops before
committing it.

We only need to set iommu ops if it is not the current ops; this
avoids searching the tree for the iommu table unnecessarily.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>