Add safety checking to dgpu_disable, egpu_enable, gpu_mux_mode.

These checks prevent users from doing such things as:
- disabling the dGPU while is muxed to drive the internal screen
- enabling the eGPU which also disables the dGPU, while muxed to
  the internal screen
- switching the MUX to dGPU while the dGPU is disabled
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230630053552.976579-7-luke@ljones.devSigned-off-by: Hans de Goede <hdegoede@redhat.com>