pad: clear probes holding mutex

Protect clearing probes against concurrent modification which might happen
due to dispose does NOT guarantee that the object is not used anymore, as
it could be referenced again and so being continued used.
So, as in the rest of places where probes hook list is used, on dispose
it should be accessed holding the mutex "GST_OBJECT_LOCK (pad);" as
GHookList is not thread-safe.

Part-of: <gstreamer/gstreamer!800>
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merge request