LF-5604 imx8m: vpu_dec: handle invalid timestamp which tv_sec or tv_usec is negative

commit  de5ba8eb72a6d143fa6e8b37c69970175511abf4 from
https://github.com/nxp-imx/linux-imx

 lf-6.1.y

If tv_usec is negative, which convert to u32 is a big positive value,
thus will confusing timestamp manager. Use 'BUF_FLAG_TIMESTAMP_INVALID'
to idicate such invalid timestamp which tv_sec or tv_usec is negative.

Signed-off-by: Shijie Qin <shijie.qin@nxp.com>
Reviewed-by: ming_qian <ming.qian@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>