media: rockchip/vpu: Prepare things to support decoders

The code in rockchip_vpu_v4l2 was hardcoded for encoder support.
Modify it more generic to support the decoder case so that we can
re-use the same vb2/v4l2 ops for both devices.

Co-developed-by: Ezequiel Garcia <ezequiel@collabora.com>

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>