screen-cast-src: Destroy hash dmabuf table after stream

The stream will clean up the buffers, so let it do that before we
destroy them under its feet. Note that it'll only do this after the
following PipeWire commit:

    commit fbaa4ddedd84afdffca16f090dcc4b0db8ccfc29
    Author: Wim Taymans <wtaymans@redhat.com>
    Date:   Mon Jun 1 15:36:09 2020 +0200

        stream: allow NULL param and 0 buffers in disconnect

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1283


(cherry picked from commit 97175f8f)