[idd] helper: load default mode list with default refresh rate

After 882d531792, the existing code in the
helper reads past the end of the array for refresh rate, which is broken.

This PR also refactors the default modes into a getDefaultModes method so
that it can be used for a reset feature later.

idd/LGIddHelper/CRegistrySettings.h

@@ -44,6 +44,7 @@ public:
   LSTATUS open();
   bool isOpen() { return !!hKey; }
 
+  std::vector<DisplayMode> getDefaultModes();
   std::optional<std::vector<DisplayMode>> getModes();
   LSTATUS setModes(const std::vector<DisplayMode> &modes);