va: Move back parent_object to each element.

va: Move back parent_object to each element.

Using GstBaseDec hack to access the parent_object of each element in
the element itself is a bit fragile. It would be better to keep its
own parent object as the usual global variable. It would make it
resistant to code changes.

The GstBaseDec macro to access the parent object now it's internal to
base decoder.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1257>