Move over to a mutex-spi to prepare for the radio

I don't like this one bit. It's very hacky, and it's subject to race conditions if the mutex is shared between priorities. So I will have to ensure the radio runs in the same priority (or more probably, in the same task). Honestly I hope this all gets ripped out if/when I redesign the board to give the display a dedicated SPI line. But this will help me continue development for now