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>

arch/sandbox/include/asm/sdl.h

@@ -25,6 +25,13 @@
 int sandbox_sdl_init_display(int width, int height, int log2_bpp,
 			     bool double_size);
 
+/**
+ * sandbox_sdl_remove_display() - Remove the SDL screen
+ *
+ * @return 0 if OK, -ENOENT if the SDL had not been inited.
+ */
+int sandbox_sdl_remove_display(void);
+
 /**
  * sandbox_sdl_sync() - Sync current U-Boot LCD frame buffer to SDL
  *