lavc/qsvenc_jpeg: add async_depth support

Currently qsv (m)jpeg encoding is broken.
Regression introducing by the commit(id: c1bcd3): fix async support,
which requires the minimum async_depth to be 1, instead previous zero.
But the default async_depth of qsv (m)jpeg encoding is still initialized
(mostly) as zero.

This patch also abviously improves qsv (m)jpeg encoding performance
due to the default async_depth is changed to 4.
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/qsvenc_jpeg.c

@@ -64,6 +64,7 @@ static av_cold int qsv_enc_close(AVCodecContext *avctx)
 #define OFFSET(x) offsetof(QSVMJPEGEncContext, x)
 #define VE AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption options[] = {
+    { "async_depth", "Maximum processing parallelism", OFFSET(qsv.async_depth), AV_OPT_TYPE_INT, { .i64 = ASYNC_DEPTH_DEFAULT }, 1, INT_MAX, VE },
     { NULL },
 };