drm/amd/amdgpu: Partial revert of iova debugfs

We discovered that on some devices even with iommu enabled
you can access all of system memory through the iommu translation.

Therefore, we revert the read method to the translation only service
and drop the write method completely.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christan König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>