[media] coda: move parameter buffer in together with context buffer allocation

The parameter buffer is a per-context buffer, so we can allocate and free it
together with the other context buffers during REQBUFS.
Since this was the last context buffer allocated in coda-common.c, we can now
move coda_alloc_context_buf into coda-bit.c.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>