avdec_h264: Always negotiate the pool with the downstream element in some solution and very bad performace.

The pipeline of:

gst-launch-1.0 -vf videotestsrc ! video/x-raw,width=800,height=600 ! vaapih264enc ! avdec_h264 ! vapostproc brightness=0.8 ! fakesink

doing the endless the pool negotiation with the downstream element,

libav gstavviddec.c:746:gst_ffmpegviddec_ensure_internal_pool:<avdec_h264-0> Updating internal pool (800, 608)

Which causes the downstream element creating/destroying the gst buffer pool all the time, and so, very low performance.