drm/mediatek: call drm_atomic_helper_shutdown() when unbinding driver

shutdown all CRTC when unbinding drm driver.

Fixes: 119f5173 ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>