[media] coda: split encoder specific parts out of device_run and irq_handler

Add coda_prepare_encode() and coda_finish_encode() functions. They are called
from coda_device_run() and coda_irq_handler(), respectively, before and after
the hardware picture run. This should make the following decoder support patch
easier to read, which will add the coda_prepare/finish_decode() equivalents.

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>