After fixing https://bugs.freedesktop.org/show_bug.cgi?id=100972 , 
ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi -i Elephants_Dream_HD.avi
-vf format=yuv420p,hwupload -threads 2 -acodec copy  -vaapi_device
/dev/dri/renderD128  -bf 0 -c:v h264_vaapi -c:v h264_vaapi -profile:v 77
~/test.mkv

seems to work (ffmpeg from git master).
Performance is atrocious though : I get about an encoding rate of about 2 FPS.