drivers/display/ssd1306: Implement SSD1306_I2C poweron method.

After a poweroff(), the poweron() method does a soft power-on and any
previous state of the display persists.

drivers/display/ssd1306.py

@@ -124,7 +124,7 @@ class SSD1306_I2C(SSD1306):
         self.i2c.stop()
 
     def poweron(self):
-        pass
+        self.write_cmd(SET_DISP | 0x01)
 
 
 class SSD1306_SPI(SSD1306):