video: add config option to skip framebuffer clear

The use-case is that the thing that loaded u-boot already put a splash
image on screen.  And we want to preserve that until grub boot menu
takes over.

Signed-off-by: Rob Clark <robdclark@gmail.com>
diff --git a/drivers/video/video-uclass.c b/drivers/video/video-uclass.c
index 3036e3a..dfa39b0 100644
--- a/drivers/video/video-uclass.c
+++ b/drivers/video/video-uclass.c
@@ -199,7 +199,9 @@
 #else
 	priv->colour_bg = 0xffffff;
 #endif
-	video_clear(dev);
+
+	if (!CONFIG_IS_ENABLED(NO_FB_CLEAR))
+		video_clear(dev);
 
 	/*
 	 * Create a text console device. For now we always do this, although