drm/amdgpu: disable system memory page tables for now

We hit a problem with IOMMU with that. Disable until we have time to
debug further.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>