feat(display): Don't send superfluous sleep commands

"Cache" the local sleep value to not send repetitive sleep-out commands
to the display when the user repeatedly sets the backlight.

epicardium/drivers/display/lcd.c

@@ -161,6 +161,13 @@ static void lcd_hard_reset(void)
 
 void lcd_set_sleep(bool sleep)
 {
+	static int current_sleep = -1;
+
+	if (sleep == current_sleep) {
+		return;
+	}
+	current_sleep = sleep;
+
 	if (sleep) {
 		lcd_send_command(LCD_SLPIN, NULL, 0);
 	} else {