VAAPI: Don't return false when vlVaQueryDisplayAttributes returns empty

Mesa tracker does not implement VaQueryDisplayAttributes and returns zero attributes. As we only use m_attributes for enumeration purposes, we should not return false at this point.

@FernetMenta What do you think? It's a "Starthilfe" for mesa devs, wrapper devs and so on.

Besides that the status of mesa vaapi tracker is shown here: 
![](http://abload.de/img/bildschirmfoto-07.12.mkjmj.png)