mesa/util: add allow_glsl_builtin_const_expression driconf override

Google Earth VR shaders uses builtins in constant expressions with
GLSL 1.10. That feature wasn't allowed until GLSL 1.20.

Reviewed-by: Dave Airlie <airlied@redhat.com>

src/compiler/glsl/ast_function.cpp

@@ -529,7 +529,8 @@ generate_call(exec_list *instructions, ir_function_signature *sig,
     * If the function call is a constant expression, don't generate any
     * instructions; just generate an ir_constant.
     */
-   if (state->is_version(120, 100)) {
+   if (state->is_version(120, 100) ||
+       state->ctx->Const.AllowGLSLBuiltinConstantExpression) {
       ir_constant *value = sig->constant_expression_value(ctx,
                                                           actual_parameters,
                                                           NULL);