vaapipostproc: fix detection of output surface format changes.
vaapipostproc: fix detection of output surface format changes.

Default to I420 format for output surfaces so that to match the usual
GStreamer pipelines. Though, internally, we could still opt for NV12
surface formats, i.e. default format=ENCODED is a hint for that, thus
delegating the decision to the VA driver.

gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi