Avoid disabling outputs which are already disabled

Better to avoid making unnecessary calls, but mainly we want to avoid
logging false errors.
diff --git a/unix/common/randr.cxx b/unix/common/randr.cxx
index 76ae0bc..9cfc95d 100644
--- a/unix/common/randr.cxx
+++ b/unix/common/randr.cxx
@@ -300,6 +300,10 @@
     if (outputIdMap->count(output) == 1)
       continue;
 
+    /* Enabled? */
+    if (!vncRandRIsOutputEnabled(i))
+      continue;
+
     /* Disable and move on... */
     ret = vncRandRDisableOutput(i);
     if (!ret) {