i965: Only add the wpos state reference if we lowered something

i965: Only add the wpos state reference if we lowered something

Otherwise, in the ARB program case _mesa_add_state_reference may grow
the parameter array which will cause brw_nir_setup_arb_uniforms to write
past the end of the param array because it only looks at the parameter
list length but the parma array is allocated based on nir->num_uniforms.
The only reason this hasn't caused us problems is because we are padding
out the param array for fragment programs unnecessarily.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>