drm/amdgpu: apply the RV2 system aperture fix to RN/CZN as well

These chips needs the same fix.  This was previously not seen
on then since the AGP aperture expanded the system aperture,
but this showed up again when AGP was disabled.

Reviewed-and-tested-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>