drm/amdgpu: set system aperture to cover whole FB region in mmhub v9.4

In XGMI configuration, the FB region covers vram region from peer
device, adjust system aperture to cover all of them

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>