codecs: h264decoder: Store reference picture type using enum value

codecs: h264decoder: Store reference picture type using enum value

Managing reference picture type by using two variables
(ref and long_term) seems to be redundant and that can be
represented by using a single enum value.

This is to sync this implementation with gstreamer-vaapi so that
make comparison between this and gstreamer-vaapi easier and also
in order to minimize the change required for subclass to be able
to support interlaced.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1534>