VAAPI: Don't return false when unknown driver is used - warn instead

As it seems the opensource world is moving forward to VAAPI. We should make sure, that we will run on the new AMD opensource vaapi stack, too.

This allows to open the decoder - but warns so that users with broken fglrx wrappers still clearly see in their logfiles what the issues.
Not sure about that, but also not against it. 

Most users don't read anything at all anyways - I think the warning in the log is enough - as this log they post when it crashes and we can read it.