You have a muxless systems (i.e., the laptop panel is only attached to the integrated GPU). The panel is not attached to the discrete card so there is no way to switch. To properly support muxless systems the X server needs significant architectural changes to decouple rendering and display (basically both GPUs are active and the discrete card renders into a buffer that is shared with the integrated card. The integrated card then composites that buffer with the rest of the desktop when it updates the screen. There is some work going on to support this, but it's not ready yet.
Originally Posted by dogsleg