vaapipostproc: use scoped variable for return value
vaapipostproc: use scoped variable for return value

Instead of reusing a parameter variable for the return value of
gst_vaapipostproc_fixate_caps(), this patch uses the function scoped
pointer. Thus, the code is cleaner.

https://bugzilla.gnome.org/show_bug.cgi?id=785706

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