[media] coda: add context ops

Add a struct coda_context_ops that encapsulates context specific operations.
This will simplify adding JPEG support in the future and helps to avoid
exporting all functions individually when they move out of the main code
file.

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