vah264enc: Avoid mapping coded buffer twice.

vah264enc: Avoid mapping coded buffer twice.

Currently for copying the coded buffer onto a GStreamer buffer, the
coded buffer is mapped two times: one for getting the size, and later
for do the actual copy. We can avoid this by doing directly in the
element rather than in the general encoder object.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1845>