acornfb: use display information in info not in var for panning

We must not use any information in the passed var besides xoffset,
yoffset and vmode as otherwise applications might abuse it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
diff --git a/drivers/video/acornfb.c b/drivers/video/acornfb.c
index 3bacc12..b303f17 100644
--- a/drivers/video/acornfb.c
+++ b/drivers/video/acornfb.c
@@ -850,9 +850,9 @@
 	u_int y_bottom = var->yoffset;
 
 	if (!(var->vmode & FB_VMODE_YWRAP))
-		y_bottom += var->yres;
+		y_bottom += info->var.yres;
 
-	if (y_bottom > var->yres_virtual)
+	if (y_bottom > info->var.yres_virtual)
 		return -EINVAL;
 
 	acornfb_update_dma(info, var);