glsl: Make use of new _mesa_glsl_parse_state::is_version() function.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Carl Worth <cworth@cworth.org>

src/glsl/ast_function.cpp

@@ -277,7 +277,7 @@ generate_call(exec_list *instructions, ir_function_signature *sig,
     *
     * Function calls were first allowed to be constant expressions in GLSL 1.20.
     */
-   if (state->language_version >= 120) {
+   if (state->is_version(120, 0)) {
       ir_constant *value = sig->constant_expression_value(actual_parameters, NULL);
       if (value != NULL) {
 	 return value;