iommu/amd: Do not support IOMMU_DOMAIN_IDENTITY after SNP is enabled

Once SNP is enabled (by executing SNP_INIT command), IOMMU can no longer
support the passthrough domain (i.e. IOMMU_DOMAIN_IDENTITY).

The SNP_INIT command is called early in the boot process, and would fail
if the kernel is configure to default to passthrough mode.

After the system is already booted, users can try to change IOMMU domain
type of a particular IOMMU group. In this case, the IOMMU driver needs to
check the SNP-enable status and return failure when requesting to change
domain type to identity.

Therefore, return failure when trying to allocate identity domain.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20220713225651.20758-9-suravee.suthikulpanit@amd.com
[ joro: Removed WARN_ON_ONCE() ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>