libs: filter: use macro for returning op default value
libs: filter: use macro for returning op default value

The code is essentially the same for getting all op default
values.  Thus, use a macro to help minimize code duplication
and [hopefully] encourage using the same mechanism for all
default getters.

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