Use 'ui.' where possible

Marlin/src/lcd/extensible_ui/lib/dgus/DGUSDisplayDefinition.cpp

   VPHELPER(VP_ZPos, &current_position[2], nullptr, DGUSScreenVariableHandler::DGUSLCD_SendFloatAsLongValueToDisplay<2>),
 
   // Print Progress.
-  VPHELPER(VP_PrintProgress_Percentage, &MarlinUI::progress_bar_percent, nullptr, DGUSScreenVariableHandler::DGUSLCD_SendWordValueToDisplay ),
+  VPHELPER(VP_PrintProgress_Percentage, &ui.progress_bar_percent, nullptr, DGUSScreenVariableHandler::DGUSLCD_SendWordValueToDisplay ),
 
   // Print Time
   VPHELPER_STR(VP_PrintTime, nullptr, VP_PrintTime_LEN, nullptr, DGUSScreenVariableHandler::DGUSLCD_SendPrintTimeToDisplay ),