drm/panel: st7703: Enter sleep after display off

The datasheet suggests to issue sleep in after display off
as a part of the panel's shutdown sequence.
Signed-off-by: Ondrej Jirman <megous@megous.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701162928.1638874-11-megous@megous.com

drivers/gpu/drm/panel/panel-sitronix-st7703.c

@@ -393,8 +393,19 @@ static int st7703_disable(struct drm_panel *panel)
 {
 	struct st7703 *ctx = panel_to_st7703(panel);
 	struct mipi_dsi_device *dsi = to_mipi_dsi_device(ctx->dev);
+	int ret;
+
+	ret = mipi_dsi_dcs_set_display_off(dsi);
+	if (ret < 0)
+		DRM_DEV_ERROR(ctx->dev,
+			      "Failed to turn off the display: %d\n", ret);
 
-	return mipi_dsi_dcs_set_display_off(dsi);
+	ret = mipi_dsi_dcs_enter_sleep_mode(dsi);
+	if (ret < 0)
+		DRM_DEV_ERROR(ctx->dev,
+			      "Failed to enter sleep mode: %d\n", ret);
+
+	return 0;
 }
 
 static int st7703_unprepare(struct drm_panel *panel)