drm/amdgpu/gmc9: fix VM_L2_CNTL3 programming

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