libs: encoder: vp9: Fix initialization of ref_list
libs: encoder: vp9: Fix initialization of ref_list

gcc 7.0.1 gives a memset-elt-size warning in gst_vaapi_encoder_vp9_init:

 'memset' used with length equal to number of elements without
 multiplication by element size [-Werror=memset-elt-size]

https://bugzilla.gnome.org/show_bug.cgi?id=780947

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi