media: rockchip/vpu: Cleanup JPEG bounce buffer management

In order to make the code more generic, introduce a pair of start/stop
codec operations, and use them to allocate and release the JPEG bounce
buffer.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>