dmabuf export from vaapi encoders fails

Submitted by Scott D Phillips @scott-ph

Link to original bug (#779145)

Description

Example pipeline:

v4l2src io-mode=dmabuf-import ! video/x-raw,format=YUY2 ! vaapijpegenc ! fakesink

The cause here is that the vaSurface's provided by vaapijpegenc still have the derived image existing. This causes vaBeginPicture to fail with a "surface busy" error.