drm/amdkfd: disable IOMMUv2 support for Raven

Use the dGPU path instead.  There were a lot of platform
issues with IOMMU in general on these chips due to windows
not enabling IOMMU at the time.  The dGPU path has been
used for a long time with newer APUs and works fine.  This
also paves the way to simplify the driver significantly.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>