desire helper: fix text flicker when changing direction (#28720)

* draft

* or this works

* or simply this

selfdrive/controls/lib/desire_helper.py

@@ -69,6 +69,7 @@ class DesireHelper:
 
         if not one_blinker or below_lane_change_speed:
           self.lane_change_state = LaneChangeState.off
+          self.lane_change_direction = LaneChangeDirection.none
         elif torque_applied and not blindspot_detected:
           self.lane_change_state = LaneChangeState.laneChangeStarting