hwcontext_vaapi: Return all formats for constraints without config

hwcontext_vaapi: Return all formats for constraints without config

No longer make a dummy device configuration to query.  Instead, just
return everything we recognise from the whole format list.  Also
change the device setup code to query that list only, rather than
intersecting it with the constraint output.

This makes hwupload more usable on mesa/gallium where the video
processor only declares support for RGB formats, making it unable to
deal with YUV formats before this patch.  It might introduce some
different trickier failures in the internal upload/download code
because the set of allowed formats there has changed, though I didn't
find any obvious regressions with i965.