media: mediatek: vcodec: align fw interface

Align scp and vpu firmware interface, remove the depedency for
'struct mtk_vcodec_dev' and 'struct mtk_vcodec_ctx'. It will be
much easier to separate video encoder and decoder.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>