drm/amdgpu: set system aperture to cover whole FB region

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

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: shaoyunl <shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>