drm/ttm: cleanup ttm_bo_mem_space

We tried this once before, but that turned out to be more
complicated than thought. With all the right prerequisites
it looks like we can do this now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>