Only query once for XFree86-VidModeExtension
Review Request #122272 - Created Jan. 27, 2015 and submitted
currentRefreshRate is called after each randr event and we don't
need to query the extension each time.
compile checked, not runtime checked as I don't have an nvidia setup.
It's btw. questionable whether we still require the entire xvidmode + nvidia-settings stuff since it was initally introduced because the blob faked the refresh rate towards xrandr what's no more the case for years now.