dri2: Disable when Xinerama is active

Would only work on ScreenRec 0, which means it's broken.

Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>

hw/xfree86/dri2/dri2ext.c

@@ -672,6 +672,11 @@ DRI2ExtensionInit(void)
 {
     ExtensionEntry *dri2Extension;
 
+#ifdef PANORAMIX
+    if (!noPanoramiXExtension)
+        return;
+#endif
+
     dri2Extension = AddExtension(DRI2_NAME,
                                  DRI2NumberEvents,
                                  DRI2NumberErrors,