media: cedrus: identify buffers by timestamp

Use the new v4l2_m2m_buf_copy_data helper function and use
timestamps to refer to reference frames instead of using
buffer indices.

Also remove the padding fields in the structs, that's a bad
idea. Just use the right types to keep everything aligned.

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