vapostproc: Move up color balance detection to plugin.

vapostproc: Move up color balance detection to plugin.

In order to install the color balance interface, a GstVaFilter is
instantiated and queried to know if it supports color balance
filter. It was done just after the GObject was registered. Now, it's
done before.

The reason of this change is that deinterlace element has to be
registered only if deinterlace filter is available, using only one
instantiate of GstVaFilter.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2495>