XIQueryVersion (like all other calls) makes sure XGetExtensionVersion is
called beforehand anyway. So if that doesn't match 2.0 or higher, return
BadRquest before issuing the real request (which would trigger a BadRequest
error). This way, clients can use XIQueryVersion without having to set up
the error handler.

XIQueryVersion is now guaranteed to return the server-supported version.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>