gallium: changes to polygon mode weren't detected in draw_unfilled stage.

Need to reset stage->tri = unfilled_first_try in unfilled_flush() so that the
front/back state is picked up.

src/mesa/pipe/draw/draw_unfilled.c

@@ -165,6 +165,8 @@ static void unfilled_flush( struct draw_stage *stage,
 			    unsigned flags )
 {
    stage->next->flush( stage->next, flags );
+
+   stage->tri = unfilled_first_tri;
 }