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/develop/driver-model/migration.rst b/doc/develop/driver-model/migration.rst
index 645c45b..43665de 100644
--- a/doc/develop/driver-model/migration.rst
+++ b/doc/develop/driver-model/migration.rst
@@ -80,7 +80,7 @@
 Deadline: 2019.07
 
 The video subsystem has supported driver model since early 2016. Maintainers
-should submit patches switching over to using CONFIG_DM_VIDEO and other base
+should submit patches switching over to using CONFIG_VIDEO and other base
 driver model options in time for inclusion in the 2019.07 release.
 
 CONFIG_DM_ETH