Release non-primary displays at code.py exit

[jepler]

Since #10062 we have supervisor.runtime.display which can be set from code or by the C board init function, and then accessed across multiple invocations of code.py.

As an incompatible change in CircuitPython 10, release non-primary displays when shutting down the interpreter. This is the next step towards fully implementing #8675. (Changing how i2c and spi objects used by display buses are allocated is the final, and hardest, part)