iommu: Inline iommu_group_get_for_dev() into __iommu_probe_device()

This is the only caller, and it doesn't need the generality of the
function. We already know there is no iommu_group, so it is simply two
function calls.

Moving it here allows the following patches to split the logic in these
functions.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v3-328044aa278c+45e49-iommu_probe_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>