drm/mediatek: clear num_pipes when unbind driver

num_pipes is used for mutex created in mtk_drm_crtc_create(). If we
don't clear num_pipes count, when rebinding driver, the count will
be accumulated. From mtk_disp_mutex_get(), there can only be at most
10 mutex id. Clear this number so it starts from 0 in every rebind.

Fixes: 119f517362 ("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>

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -400,6 +400,7 @@ static void mtk_drm_unbind(struct device *dev)
 	drm_dev_unregister(private->drm);
 	mtk_drm_kms_deinit(private->drm);
 	drm_dev_put(private->drm);
+	private->num_pipes = 0;
 	private->drm = NULL;
 }