drm/amdgpu: enable AGP aperture for GMC9 v2

Enable the old AGP aperture to avoid GART mappings.

v2: don't enable it for SRIOV

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>