media: coda: add coda_slice_mode() function

Changing slice mode dynamically while encoding will require to calculate
the register value again, so split it out into a separate function.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>