video: Rename CONFIG_DM_VIDEO to CONFIG_VIDEO

Now that all the old code is gone, rename this option. Driver model
migration is now complete.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
index d12bbcd..15897f6 100644
--- a/doc/usage/environment.rst
+++ b/doc/usage/environment.rst
@@ -63,7 +63,7 @@
 Example::
 
     stdout=serial
-    #ifdef CONFIG_DM_VIDEO
+    #ifdef CONFIG_VIDEO
     stdout+=,vidconsole
     #endif
     bootcmd=