iommu: Don't print warning when IOMMU driver only supports unmanaged domains

Print the warning about the fall-back to IOMMU_DOMAIN_DMA in
iommu_group_get_for_dev() only when such a domain was
actually allocated.

Otherwise the user will get misleading warnings in the
kernel log when the iommu driver used doesn't support
IOMMU_DOMAIN_DMA and IOMMU_DOMAIN_IDENTITY.

Fixes: fccb4e3b

 ('iommu: Allow default domain type to be set on the kernel command line')
Signed-off-by: Joerg Roedel <jroedel@suse.de>