media: cedrus: Allow using the current dst buffer as reference

It was reported that some cases of interleaved video decoding require
using the current destination buffer as a reference. However, this is
no longer possible after the move to vb2_find_timestamp because only
dequeued and done buffers are considered.

Add a helper in our driver that also considers the current destination
buffer before resorting to vb2_find_timestamp and use it in MPEG-2.

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