powerpc/iommu: Setup a default domain and remove set_platform_dma_ops

POWER is using the set_platform_dma_ops() callback to hook up its private
dma_ops, but this is buired under some indirection and is weirdly
happening for a BLOCKED domain as well.

For better documentation create a PLATFORM domain to manage the dma_ops,
since that is what it is for, and make the BLOCKED domain an alias for
it. BLOCKED is required for VFIO.

Also removes the leaky allocation of the BLOCKED domain by using a global
static.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v8-81230027b2fa+9d-iommu_all_defdom_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>