drm: replace drm_modeset_lock_all() in drm_client_modeset_dpms_legacy()

This patch helps complete Use  DRM_MODESET_LOCK_ALL* helpers instead of
boilerplate todo in Documentation/gpu/todo.rst
Signed-off-by: Joseph Schulte <joeschulte.js@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125144815.8389-1-joeschulte.js@gmail.com
(cherry picked from commit 482f3ef0)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/drm_client_modeset.c

@@ -7,6 +7,7 @@
  * Copyright (c) 2007 Dave Airlie <airlied@linux.ie>
  */
 
+#include "drm/drm_modeset_lock.h"
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/slab.h>
@@ -1181,9 +1182,11 @@ static void drm_client_modeset_dpms_legacy(struct drm_client_dev *client, int dp
 	struct drm_device *dev = client->dev;
 	struct drm_connector *connector;
 	struct drm_mode_set *modeset;
+	struct drm_modeset_acquire_ctx ctx;
 	int j;
+	int ret;
 
-	drm_modeset_lock_all(dev);
+	DRM_MODESET_LOCK_ALL_BEGIN(dev, ctx, 0, ret);
 	drm_client_for_each_modeset(modeset, client) {
 		if (!modeset->crtc->enabled)
 			continue;
@@ -1195,7 +1198,7 @@ static void drm_client_modeset_dpms_legacy(struct drm_client_dev *client, int dp
 				dev->mode_config.dpms_property, dpms_mode);
 		}
 	}
-	drm_modeset_unlock_all(dev);
+	DRM_MODESET_LOCK_ALL_END(dev, ctx, ret);
 }
 
 /**