Picking a power supply for a TMC2130 stepper driver and motor
The important thing to look at here is the first wattage calculation you did. The 12.8W that the motor will consume is the max wattage you will ever want to drive the motor (regardless of power supply voltage). 

Think about it like this. If you where to connect that 24V power supply directly to the motor it would probably be a very smokey experience. But that doesn't mean we cant use a 24V power supply. We now just need to limit the current by some proportion to make sure that the motor doesn't get more than 12.8W. 

To do this the TMC2130 has an AIN pin that allows you to reduce the current going to the motor. I believe this device also allows you to do this digitally as well. 

So to answer your question you can use either a 12V or 24V supply. You will just need to set the AIN so that you are using about 10%  (@24V) of the current the motor would typically draw at that voltage.