drm/mediatek: fix unbind functions

detatch panel in mtk_dsi_destroy_conn_enc(), since .bind will try to
attach it again.

Fixes: 2e54c14e ("drm/mediatek: Add DSI sub driver")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>