powerpc/iommu: Refactor spapr_tce_platform_iommu_attach_dev()

The patch makes the iommu_group_get() call only when using it
thereby avoiding the unnecessary get & put for domain already
being set case.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/170800513841.2411.13524607664262048895.stgit@linux.ibm.com