Update matrix_11x7.py

The new formula does not give the same result as the previous one, so it is both corrected and "optimized".
With previous formula, the screen was mirror upside down. That was visible using the font scrolling demo.
Also tested with display.pixel(0,0,50) that light up the top left pixel.

All the tests were performed by Michael Horne: https://twitter.com/recantha/status/1369276823620751362?s=20

Author: dglaude

adafruit_is31fl3731/matrix_11x7.py

@@ -42,8 +42,4 @@ def __init__(self, i2c, address=0x75):
     @staticmethod
     def pixel_addr(x, y):
         """Translate an x,y coordinate to a pixel index."""
-        if x <= 5:
-            r = x * 16 + y
-        else:
-            r = (x - 5) * 16 + y - 8
-        return r
+        return (x << 4) - y + (6 if x <= 5 else -82)