[media] coda: add workqueue to serialize hardware commands

Using the coda_mutex lock to serialize hardware access would cause
"INFO: possible circular locking dependency detected" lockdep warnings.
Since the possible locking paths are hard to follow, serialize hardware
access with a single workqueue thread. Ultimately the workqueue could
be converted to only do register setup and readout for per-command work
items.
Using the initialized context property, SEQ_END is only queued in
coda_release when needed.

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>