[media] coda: move per-instance buffer allocation and cleanup

This patch moves the context buffer allocation into the context start_streaming
callbacks. The context buffer and internal framebuffer cleanup is moved into
the context release callback.

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>