This can avoid to exhaust the buffers within outlink when libva2 is
available.

For example:
$ ffmpeg -hwaccel_output_format vaapi -hwaccel vaapi -i input.mp4 \
-vf 'scale_vaapi=w=720:h=480' -c:v hevc_vaapi -f null -
...
[vf#0:0 @ 0x55acad91f400] Error while filtering: Cannot allocate memory
[vf#0:0 @ 0x55acad91f400] Task finished with error code: -12 (Cannot
allocate memory)
[vf#0:0 @ 0x55acad91f400] Terminating thread with return code -12
(Cannot allocate memory)
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>