mesa/st: always lower psiz for vertex stages

while this is not the "most correct"™ method of running the pass, failing to run the pass during precompile is effectively generating unusable shaders in 100% of cases, as the driver will always need this, meaning that the precompile is nothing but wasted cycles

unless/until the primitive mode starts being used in the mesa/st shader variant key, drivers can determine on their own whether to eliminate the psiz output