drm/mcde: Fix uninitialized value

"val" isn't initialized on the default: errorpath.
Just return from the function if this happens.
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119140707.1008407-1-linus.walleij@linaro.org

drivers/gpu/drm/mcde/mcde_display.c

@@ -569,7 +569,7 @@ static void mcde_configure_channel(struct mcde *mcde, enum mcde_channel ch,
 	default:
 		dev_err(mcde->dev, "unknown flow mode %d\n",
 			mcde->flow_mode);
-		break;
+		return;
 	}
 
 	writel(val, mcde->regs + sync);