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

As Laurent pointed out we must not use any information in the passed
var besides xoffset, yoffset and vmode as otherwise applications
might abuse it. Also use the aligned fix.line_length and not the
(possible) unaligned xres_virtual.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@kernel.org

drivers/video/via/viafbdev.c

@@ -348,8 +348,9 @@ static int viafb_pan_display(struct fb_var_screeninfo *var,
 	struct fb_info *info)
 {
 	struct viafb_par *viapar = info->par;
-	u32 vram_addr = (var->yoffset * var->xres_virtual + var->xoffset)
-		* (var->bits_per_pixel / 8) + viapar->vram_addr;
+	u32 vram_addr = viapar->vram_addr
+		+ var->yoffset * info->fix.line_length
+		+ var->xoffset * info->var.bits_per_pixel / 8;
 
 	DEBUG_MSG(KERN_DEBUG "viafb_pan_display, address = %d\n", vram_addr);
 	if (!viafb_dual_fb) {