media: videobuf2: Introduce vb2_find_buffer()

All users of vb2_find_timestamp() combine it with vb2_get_buffer()
to retrieve a videobuf2 buffer, given a u64 timestamp.

Introduce an API for this use-case. Users will be converted to the new
API as follow-up commits.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>