video: Use enum with video_index_to_colour()

Use the provided enum with this function, so it is clearer what should be
passed to it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/video-uclass.c b/drivers/video/video-uclass.c
index d304e92..95e874b 100644
--- a/drivers/video/video-uclass.c
+++ b/drivers/video/video-uclass.c
@@ -260,7 +260,7 @@
 	{ 0xff, 0xff, 0xff },  /* white */
 };
 
-u32 video_index_to_colour(struct video_priv *priv, unsigned int idx)
+u32 video_index_to_colour(struct video_priv *priv, enum colour_idx idx)
 {
 	switch (priv->bpix) {
 	case VIDEO_BPP16: