[media] coda: add coda_encode_header helper function

In preparation for CODA7541 and CODA960 multi-stream support and for
replacement of the completion with a mutex lock, consolidate the
header encoding in a helper function.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>