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: 
![bildschirmfoto-07 12 mkjmj](https://cloud.githubusercontent.com/assets/1089309/5332501/eb052ad2-7e58-11e4-9133-c457a9ed45c2.png)

Here is the mesa implementation: http://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/state_trackers/va/display.c#n31