Allocate a GArray which is used to fill
VAPictureParameterBufferH264.ReferenceFrames (called per frame),
instead of alloc/free per frame.
Also this commit is to fix the condition where long-term reference
picture is needed for VAPictureParameterBufferH264.ReferenceFrames
entry.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1813>