RGBMatrix: fix memoryview(matrix)
RGBMatrix: fix memoryview(matrix)

Typical test:
```python
import displayio
import rgbmatrix
import board
displayio.release_displays()
matrix = rgbmatrix.RGBMatrix(
    width=128, bit_depth=4,
    rgb_pins=[board.GP0, board.GP1, board.GP2, board.GP3, board.GP4, board.GP5],
    addr_pins=[board.GP6, board.GP7, board.GP8, board.GP9],
    clock_pin=board.GP10, latch_pin=board.GP11, output_enable_pin=board.GP12)
mem = memoryview(matrix)
mem[0] = 65535 #  OK
mem[0] = 65536 #  errors (out of range)
```

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython