glvideomixer: swap control binding proxy

glvideomixer: swap control binding proxy

The ref_object and object parameters were the wrong way around.

For the typical use case where an application is setting a
GstControlBinding on the returned ghost pad:
1. our control binding would be removed when the new one was set
2. sync_values calls were not being forwarded from the internal
   pad to the ghost pad.

If an application attempts to perform other control binding
operations (get_* family of functions) on the internal pad, they
will also be forwarded to the ghost pad where a possible
GstControlBinding will provide the necessary values.