lavc/qsvenc_jpeg: add async_depth support

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>