i965: Silence one more compile warning.

We don't want to store this thing in the class, and we do need the
definition to be at the top of the function and held onto until the end
here, so there's not much to do besides (void) reference it.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -892,6 +892,7 @@ fs_visitor::setup_uniform_values(ir_variable *ir)
    /* Make sure we actually initialized the right amount of stuff here. */
    assert(params_before + ir->type->component_slots() ==
           c->prog_data.nr_params);
+   (void)params_before;
 }