Flickering when adjusting board.DISPLAY.brightness

This test program should smoothly increase the brightness, but there's prounounced flickering as the brightness increases. Flickering is visible both on PyPortal and Hallowing:

```python
import time
import board
import displayio

while True:
    # Fade up the backlight
    for b in range(101):
        board.DISPLAY.brightness = b / 100
        time.sleep(0.1)
```
