drm/amdgpu/gmc9: fix VM_L2_CNTL3 programming

commit 1925e7d3d4677e681cc2e878c2bdbeaee988c8e2 upstream.

Got accidently dropped when 2+1 level support was added.

Fixes: 6a42fd6f ("drm/amdgpu: implement 2+1 PD support for Raven v3")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>