drm/panel: jh057n00900: Use drm_panel_{unprepare, disable} consistently

We were already using the generic functions in our debugfs code, do the
same in jh057n_shutdown. This was suggested by Sam Ravnborg.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/a37dd5083462064f437ff62fd84e6576d8a7c8dc.1564146727.git.agx@sigxcpu.org

drivers/gpu/drm/panel/panel-rocktech-jh057n00900.c

@@ -372,12 +372,12 @@ static void jh057n_shutdown(struct mipi_dsi_device *dsi)
 	struct jh057n *ctx = mipi_dsi_get_drvdata(dsi);
 	int ret;
 
-	ret = jh057n_unprepare(&ctx->panel);
+	ret = drm_panel_unprepare(&ctx->panel);
 	if (ret < 0)
 		DRM_DEV_ERROR(&dsi->dev, "Failed to unprepare panel: %d\n",
 			      ret);
 
-	ret = jh057n_disable(&ctx->panel);
+	ret = drm_panel_disable(&ctx->panel);
 	if (ret < 0)
 		DRM_DEV_ERROR(&dsi->dev, "Failed to disable panel: %d\n",
 			      ret);