omxbufferpool: reference the OMX component
omxbufferpool: reference the OMX component

Now that the pool is responsible of freeing the OMX buffers, we need to
ensure that the OMX component stay alive while the pool is as we rely on
the component to free the buffers.
The GstOMXPort is owned by the component so no need to ref this one.

https://bugzilla.gnome.org/show_bug.cgi?id=796918

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx