media: cedrus: Use vb2_find_buffer

Use the newly introduced vb2_find_buffer API to get a vb2_buffer
given a buffer timestamp.

Cc: Maxime Ripard <mripard@kernel.org>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>