st/mesa: lower global vars to local after lowering clip
When this code was merged, this wasn't necessary because the state-tracker would do it later anyway. But this recently got changed, without changing the code that depended on this.
Arguably, this was a mistake in the lowering pass to begin with. Either way, let's fix it by not assuming that the lowering code gets called later when it's not needed.
This fixed user-defined clip-planes in Zink.