video: sandbox: Avoid duplicate display windows

When unit tests are run they currently create a new window. Update the
code so that the old one is removed first. This avoids the confusion as to
which one is active.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/sandbox_sdl.c b/drivers/video/sandbox_sdl.c
index 32739de..6e430b2 100644
--- a/drivers/video/sandbox_sdl.c
+++ b/drivers/video/sandbox_sdl.c
@@ -63,6 +63,20 @@
 		uc_plat->size *= 2;
 }
 
+static int sandbox_sdl_remove(struct udevice *dev)
+{
+	/*
+	 * Removing the display it a bit annoying when running unit tests, since
+	 * they remove all devices. It is nice to be able to see what the test
+	 * wrote onto the display. So this comment is just here to show how to
+	 * do it, if we want to make it optional one day.
+	 *
+	 * sandbox_sdl_remove_display();
+	 */
+
+	return 0;
+}
+
 static int sandbox_sdl_bind(struct udevice *dev)
 {
 	struct sandbox_sdl_plat *plat = dev_get_plat(dev);
@@ -90,5 +104,6 @@
 	.of_match = sandbox_sdl_ids,
 	.bind	= sandbox_sdl_bind,
 	.probe	= sandbox_sdl_probe,
+	.remove	= sandbox_sdl_remove,
 	.plat_auto	= sizeof(struct sandbox_sdl_plat),
 };