iommu: Move def_domain type check for untrusted device into core

iommu: Move def_domain type check for untrusted device into core

So that the vendor iommu drivers are no more required to provide the
def_domain_type callback to always isolate the untrusted devices.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
Link: https://lore.kernel.org/linux-iommu/243ce89c33fe4b9da4c56ba35acebf81@huawei.com/
Link: https://lore.kernel.org/r/20201124130604.2912899-2-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>