use python implementation for backlight (for now)

src/trezor/ui/__init__.py

@@ -7,6 +7,17 @@ from trezor import loop
 
 display = Display()
 
+# workaround for missing display.backlight in stmhal
+def backlight(val: int):
+    import sys
+    if sys.platform == 'trezor':
+        import pyb
+        timer = pyb.Timer(1, freq=1000)
+        val = max(0, min(100, val * 100 // 255))
+        timer.channel(1, pyb.Timer.PWM_INVERTED, pin=pyb.Pin.board.LCD_PWM, pulse_width_percent=val)
+    else:
+        display.backlight(val)
+
 
 def rgbcolor(r: int, g: int, b: int) -> int:
     return ((r & 0xF8) << 8) | ((g & 0xFC) << 3) | ((b & 0xF8) >> 3)