omxvideodec: unref allocator after getting it from allocation query
omxvideodec: unref allocator after getting it from allocation query

Otherwise a reference will be leaked for each allocator. It only happens
when target platform is Raspberry Pi and when we have GL support.

https://bugzilla.gnome.org/show_bug.cgi?id=751867

GStreamer/gst-omx,GStreamer/gst-omx,varunkumara/test-repo,CapOM/gst-omx,CapOM/gst-omx,CapOM/gst-omx,GStreamer/gst-omx,varunkumara/test-repo,varunkumara/test-repo