Prevent being able to change video adapters during emulation

Title
I think this should be done in `OnUpdateUI` instead. In any case, please make sure that this actually works for the following workflows:
a) start Dolphin, start emulation, open graphics config
b) start Dolphin, open graphics config
c) start Dolphin, start emulation, open graphics config immediately after having pushed the "start emulation" button (point being to check for race conditions)