[media] rcar_vin: move buffer management to .stop_streaming handler

This commit moves the "buffer in use" logic from the .buf_cleanup
handler into .stop_streaming, based on advice that this is its
proper logical home.

By ensuring the list of pointers in priv->queue_buf[] is managed
as soon as possible, we avoid warnings concerning buffers in ACTIVE
state when the system cleans up after streaming stops. This fixes a
problem with modification of buffers after their content has been
cleared for passing to userspace.

After the refactoring, the buf_init and buf_cleanup functions were
found to contain only initialisation/release steps as are carried out
elsewhere if omitted; these functions and references were removed.

Signed-off-by: William Towle <william.towle@codethink.co.uk>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>