Preserve power during PP by turning off backlight

Firmware/Marlin_main.cpp

@@ -9534,6 +9534,11 @@ void uvlo_()
 	unsigned long time_start = _millis();
 	bool sd_print = card.sdprinting;
     // Conserve power as soon as possible.
+#ifdef LCD_BL_PIN
+    backlightMode = BACKLIGHT_MODE_DIM;
+    backlightLevel_LOW = 0;
+    backlight_update();
+#endif //LCD_BL_PIN
     disable_x();
     disable_y();