drm/amdgpu: put gtt at 0 in the internal address space

There still seem to be some blocks that make accesses
in the lower part of the address space.  This works around
this.

Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>