Start to use the old AGP aperture for system memory access.

v2: Move that to amdgpu_ttm_alloc_gart
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>