Otherwise the output format is not changed when output is in system
memory. For example, the output format is still p010le in the following
case:

$ ffmpeg -qsv_device /dev/dri/renderD128 -f lavfi -i testsrc -vf
"format=p010le,vpp_qsv=extra_hw_frames=8:format=nv12" -f null -
...
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf60.4.100
  Stream #0:0: Video: wrapped_avframe, p010le(tv, progressive), 320x240
[SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>