media: mediatek: vcodec: Fix encoder access NULL pointer

Need to set the private data with encoder device, or will access
NULL pointer in encoder handler.

Fixes: 1972e324

 ("media: mediatek: vcodec: Fix possible invalid memory access for encoder")

Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>