misync: Don't smash custom screen sync functions

There was a check to avoid smashing custom functions, but the sense
was backwards causing it to always smash them, and also not set them otherwise.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

miext/sync/misync.c

@@ -190,7 +190,7 @@ miSyncSetup(ScreenPtr pScreen)
 
     pScreenPriv = SYNC_SCREEN_PRIV(pScreen);
 
-    if (pScreenPriv->funcs.CreateFence) {
+    if (!pScreenPriv->funcs.CreateFence) {
         pScreenPriv->funcs = miSyncScreenFuncs;
 
         /* Wrap CloseScreen to clean up */