media: mediatek: vcodec: separate struct 'mtk_vcodec_ctx'

Adding different context struct for encoder and decoder and removing
struct 'mtk_vcodec_ctx'.

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>