mtk-jpegenc: export jpeg encoder functions

mtk jpeg encoder is built as a module, export some functions to make them
visible by other modules.

Signed-off-by: kyrie wu <kyrie.wu@mediatek.com>
Signed-off-by: irui wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>