lavfi/qsvvpp: support async depth

lavfi/qsvvpp: support async depth

Async depth will allow qsv filter cache few frames, and avoid force
switch and end filter task frame by frame. This change will improve
performance for some multi-task case, for example 1:N transcode(
decode + vpp + encode) with all QSV plugins.

Performance data test on my Coffee Lake Desktop(i7-8700K) by using
the following 1:8 transcode test case improvement:
1. Fps improved from 55 to 130.
2. Render/Video usage improved from ~61%/~38% to ~100%/~70%.(Data get
from intel_gpu_top)

test CMD:
ffmpeg -v verbose -init_hw_device qsv=hw:/dev/dri/renderD128 -filter_hw_device                 \
 hw -hwaccel qsv -hwaccel_output_format qsv -c:v h264_qsv -i 1920x1080.264                     \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null -

Signed-off-by: Fei Wang <fei.w.wang@intel.com>
Reviewed-by: Linjie Fu <linjie.justin.fu@gmail.com>
Signed-off-by: Zhong Li <zhongli_dev@126.com>