DGA: Fix ProcXF86DGASetViewPort for missing support in driver.

Fixes a segfault when trying to activate a DGA mode without checking
whether DGA modesetting is at all possible.

hw/xfree86/dixmods/extmod/xf86dga.c

@@ -153,6 +153,9 @@ ProcXF86DGASetViewPort(ClientPtr client)
 
     REQUEST_SIZE_MATCH(xXF86DGASetViewPortReq);
 
+    if (!DGAAvailable(stuff->screen))
+	return (DGAErrorBase + XF86DGANoDirectVideoMode);
+
     if (!DGAActive(stuff->screen))
     {
 	int num;