media: amphion: add lock around vdec_g_fmt

the capture format may be changed when
sequence header is parsed,
it may be read and write in the same time,
add lock around vdec_g_fmt to synchronize it

Fixes: 6de8d628 ("media: amphion: add v4l2 m2m vpu decoder stateful driver")
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>