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/board/beckhoff/mx53cx9020/Makefile b/board/beckhoff/mx53cx9020/Makefile
index 7f15fc5..423a553 100644
--- a/board/beckhoff/mx53cx9020/Makefile
+++ b/board/beckhoff/mx53cx9020/Makefile
@@ -4,4 +4,4 @@
 # Patrick Bruenn <p.bruenn@beckhoff.com>
 
 obj-y               += mx53cx9020.o
-obj-$(CONFIG_DM_VIDEO) += mx53cx9020_video.o
+obj-$(CONFIG_VIDEO) += mx53cx9020_video.o