In fact, the va decoder is just a internal helper class and its access
is under the control of all dec elements. So far, there is no parallel
operation on it now.
At the other side, some code scan tools report race condition issues.
For example, the "context" field is just protected with lock at _open()
but is not protected at _add_param_buffer().
So we just delete all its lock usage.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7547>
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