drm/mediatek: fix unbind functions

[ Upstream commit 8fd7a37b191f93737f6280a9b5de65f98acc12c9 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>