st/mesa: enable state var merging to improve fetch_state performance
The requirement is that drivers must implement finalize_nir for st_finalize_nir to be invoked at link time and before shader variant generation. Without that, _mesa_optimize_state_parameters won't be called. Reviewed-by:Zoltán Böszörményi <zboszor@gmail.com> Part-of: <mesa/mesa!8183>