media: vcodec: Implement manual request completion

Rework how requests are completed in the MediaTek VCodec driver, by
implementing the new manual request completion feature, which allows to
keep a request open while allowing to add new bitstream data.
This is useful in this case, because the hardware has a LAT and a core
decode work, after the LAT decode the bitstream isn't required anymore
so the source buffer can be set done and the request stays open until
the core decode work finishes.

Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>