iommu/amd: Fix amd_iommu=force_isolation

The parameter is still there but it's ignored. We need to check its
value before deciding to go into passthrough mode for AMD IOMMU v2
capable device.

We occasionally use this parameter to force v2 capable device into
translation mode to debug memory corruption that we suspect is
caused by DMA writes.

To address the following comment from Joerg Roedel on the first
version, v2 capability of device is completely ignored.
> This breaks the iommu_v2 use-case, as it needs a direct mapping for the
> devices that support it.

And from Documentation/admin-guide/kernel-parameters.txt:
  This option does not override iommu=pt

Fixes: aafd8ba0

 ("iommu/amd: Implement add_device and remove_device")

Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>