expo: Add a function to prepare a cedit

Split out the code which prepares the cedit for use, so we can call it
from a test.

Add a log category while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/cedit.h

@@ -8,6 +8,7 @@
 #define __CEDIT_H
 
 struct expo;
+struct scene;
 struct video_priv;
 
 /**
@@ -30,4 +31,18 @@ int cedit_arange(struct expo *exp, struct video_priv *vid_priv, uint scene_id);
  */
 int cedit_run(struct expo *exp);
 
+/**
+ * cedit_prepare() - Prepare to run a cedit
+ *
+ * Set up the video device, select the first scene and highlight the first item.
+ * This ensures that all menus have a selected item.
+ *
+ * @exp: Expo to use
+ * @vid_privp: Set to private data for the video device
+ * @scnp: Set to the first scene
+ * Return: scene ID of first scene if OK, -ve on error
+ */
+int cedit_prepare(struct expo *exp, struct video_priv **vid_privp,
+		  struct scene **scnp);
+
 #endif /* __CEDIT_H */