Enabled one thread per plane, used the test command for 1080P video
(YUV420P format) as follow:

ffmpeg -i 1080p.mp4 -an -vf hqdn3d -f null /dev/nul

This optimization improved the performance about 30% in 1080P YUV420P
case (from 110fps to 143fps), also pass the framemd5 check and FATE.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Moritz Barsnick <barsnick@gmx.net>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>