media: amphion: decoder copy timestamp from output to capture

copy the timestamp using the helper function
V4L2_BUF_FLAG_TIMESTAMP_COPY

To implement this, driver will keep the output buffer until it's
decoded, in previous, driver will return the output buffer immediately
after copying data to stream buffer.

After that, there is no need to make a workaround for poll function.
driver can use v4l2_m2m_fop_poll directly.
Also, driver don't need to keep a input threshold
as the buffer count is up to only 32.

Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>