iommu: Move default domain allocation to separate function

Move the code out of iommu_group_get_for_dev() into a separate
function.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20200429133712.31431-2-joro@8bytes.org


Signed-off-by: Joerg Roedel <jroedel@suse.de>