drm/exynos: Fix locking in the suspend/resume paths

Commit 48a92916729b ("drm/exynos: use drm_for_each_connector_iter()")
replaced unsafe drm_for_each_connector() with drm_for_each_connector_iter()
and removed surrounding drm_modeset_lock calls. However, that lock was
there not only to protect unsafe drm_for_each_connector(), but it was also
required to be held by the dpms code which was called from the loop body.
This patch restores those drm_modeset_lock calls to fix broken suspend
and resume of Exynos DRM subsystem in v4.13 kernel.

Fixes: 48a92916729b ("drm/exynos: use drm_for_each_connector_iter()")
CC: stable@vger.kernel.org # v4.13
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_drv.c

@@ -174,6 +174,7 @@ static int exynos_drm_suspend(struct device *dev)
 	if (pm_runtime_suspended(dev) || !drm_dev)
 		return 0;
 
+	drm_modeset_lock_all(drm_dev);
 	drm_connector_list_iter_begin(drm_dev, &conn_iter);
 	drm_for_each_connector_iter(connector, &conn_iter) {
 		int old_dpms = connector->dpms;
@@ -185,6 +186,7 @@ static int exynos_drm_suspend(struct device *dev)
 		connector->dpms = old_dpms;
 	}
 	drm_connector_list_iter_end(&conn_iter);
+	drm_modeset_unlock_all(drm_dev);
 
 	return 0;
 }
@@ -198,6 +200,7 @@ static int exynos_drm_resume(struct device *dev)
 	if (pm_runtime_suspended(dev) || !drm_dev)
 		return 0;
 
+	drm_modeset_lock_all(drm_dev);
 	drm_connector_list_iter_begin(drm_dev, &conn_iter);
 	drm_for_each_connector_iter(connector, &conn_iter) {
 		if (connector->funcs->dpms) {
@@ -208,6 +211,7 @@ static int exynos_drm_resume(struct device *dev)
 		}
 	}
 	drm_connector_list_iter_end(&conn_iter);
+	drm_modeset_unlock_all(drm_dev);
 
 	return 0;
 }