media: cedrus: Remove completed item from TODO list (dma-buf references)

Access to reference frames that were imported from dma-buf was taken
care of and is no longer a pending item on the driver's TODO list.

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>