fix draw_idle reference in  NavigationToolbar2

NavigationToolbar2 sub-classes do not (all?) have a draw_idle method (strictly speaking, the QT one does not).  I assume this should have bounce strait to the canvas, not through the `NavigationToolbar2.draw()` method.

Change introduced in 25ff2f70764b71c7126929e0ef08e7b18c42a0d4