We still have a few iommu issues which need to address, so force raven
as "dgpu" path for the moment.

This is to add the fallback path to bypass IOMMU if IOMMU v2 is disabled
or ACPI CRAT table not correct.

v2: Use ignore_crat parameter to decide whether it will go with IOMMUv2.
v3: Align with existed thunk, don't change the way of raven, only renoir
    will use "dgpu" path by default.
v4: don't update global ignore_crat in the driver, and revise fallback
    function if CRAT is broken.
v5: refine acpi crat good but no iommu support case, and rename the
    title.
v6: fix the issue of dGPU initialized firstly, just modify the report
    value in the node_show().
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>