Provide a good default contrast

In reference to #15139

Marlin/src/lcd/dogm/ultralcd_DOGM.cpp

 
 #if HAS_LCD_CONTRAST
 
-  int16_t MarlinUI::contrast; // Initialized by settings.load()
+  int16_t MarlinUI::contrast = DEFAULT_LCD_CONTRAST;
 
   void MarlinUI::set_contrast(const int16_t value) {
     contrast = constrain(value, LCD_CONTRAST_MIN, LCD_CONTRAST_MAX);