[media] coda: store per-context work buffer size in struct coda_devtype

We had the workbuf_size field since the beginning.
Use it to tighten the code a little bit.

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>