plugins: check dmabuf-import for sink pad allocator
plugins: check dmabuf-import for sink pad allocator

Check earlier if upstream video source has activated the dmabuf-import
io-mode (hack to disappear soon), thus we can avoid the re-assignation of a
new allocator.

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi