video: Enable the cls command by default

This is enabled for LCD but not for VIDEO. Enable it since it is useful
to be able to clear the screen and adds very little code.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 51dbbf8..5419716 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1939,7 +1939,7 @@
 
 config CMD_CLS
 	bool "Enable clear screen command 'cls'"
-	default y if LCD
+	default y if LCD || DM_VIDEO
 	help
 	  Enable the 'cls' command which clears the screen contents
 	  on video frame buffer.