media: vb2: keep track of timestamp status

If a stream is stopped, or if a USERPTR/DMABUF buffer is queued
backed by a different user address or dmabuf fd, then the timestamp
should be skipped by vb2_find_timestamp since the memory it refers
to is no longer valid.

So keep track of a 'copied_timestamp' state: it is set when the
timestamp is copied from an output to a capture buffer, and is
cleared when it is no longer valid.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>