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

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