libs: encoders: use GST_PARAM_USER_SHIFT to define internal params
This patch makes use of GST_PARAM_USER_SHIFT to define the internal param in encoders to decide which parameters to expose. Thus gstreamer-vaapi will not interfere with any change in GStreamer in the future.
Also, the internal symbol was change to GST_VAAPI_PARAM_ENCODER_EXPOSURE to keep the namespacing.