drm/mxsfb: Add PM_SLEEP support

Since power to the lcdif block can be lost on suspend implement
PM_SLEEP_OPS using drm_mode_config_helper_suspend/resume to save/restore
the current mode.
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/cfa1a4083eefd112362e640deeb2e120584ac3f5.1537191359.git.leonard.crestez@nxp.com

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -418,6 +418,26 @@ static int mxsfb_remove(struct platform_device *pdev)
 	return 0;
 }
 
+#ifdef CONFIG_PM_SLEEP
+static int mxsfb_suspend(struct device *dev)
+{
+	struct drm_device *drm = dev_get_drvdata(dev);
+
+	return drm_mode_config_helper_suspend(drm);
+}
+
+static int mxsfb_resume(struct device *dev)
+{
+	struct drm_device *drm = dev_get_drvdata(dev);
+
+	return drm_mode_config_helper_resume(drm);
+}
+#endif
+
+static const struct dev_pm_ops mxsfb_pm_ops = {
+	SET_SYSTEM_SLEEP_PM_OPS(mxsfb_suspend, mxsfb_resume)
+};
+
 static struct platform_driver mxsfb_platform_driver = {
 	.probe		= mxsfb_probe,
 	.remove		= mxsfb_remove,
@@ -425,6 +445,7 @@ static struct platform_driver mxsfb_platform_driver = {
 	.driver	= {
 		.name		= "mxsfb",
 		.of_match_table	= mxsfb_dt_ids,
+		.pm		= &mxsfb_pm_ops,
 	},
 };