iommu: Don't overwrite domain pointer when there is no default_domain

IOMMU drivers that do not support default domains, but make
use of the the group->domain pointer can get that pointer
overwritten with NULL on device add/remove.

Make sure this can't happen by only overwriting the domain
pointer when it is NULL.

Cc: stable@vger.kernel.org # v4.4+
Fixes: 1228236d ('iommu: Move default domain allocation to iommu_group_get_for_dev()')
Signed-off-by: Joerg Roedel <jroedel@suse.de>