[media] coda: move cache setup into coda9_set_frame_cache, also use it in start_encoding

The frame cache should be set up correctly to encode NV12 source frames.
This was not done before, so move the cache setup out of start_decoding
into its own function and call it from both start_encoding and
start_decoding.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>