powerpc/pseries/iommu: Cleanup ddw naming

When using a property refering to the availibily of dynamic dma windows
call it ddw_avail not ddr_avail.

dupe_ddw_if_already_created does not dupilcate anything, it only finds
and reuses the windows we already created, so rename it to
find_existing_ddw.  Also, it does not need the pci device node, so
remove that argument.

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>