media: vb2: vb2_find_timestamp: drop restriction on buffer state

There really is no reason why vb2_find_timestamp can't just find
buffers in any state. Drop that part of the test.

This also means that vb->timestamp should only be set to 0 when
the driver doesn't copy timestamps.

This change allows for more efficient pipelining (i.e. you can use
a buffer for a reference frame even when it is queued).

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