xorg: fix exa finish access

The exa core will already set the pointer to NULL prior calling
the callback function. So don't bail out in the callback if it's
already NULL.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>

src/gallium/state_trackers/xorg/xorg_exa.c

@@ -318,7 +318,7 @@ ExaFinishAccess(PixmapPtr pPix, int index)
     if (!priv)
 	return;
 
-    if (!priv->map_transfer || pPix->devPrivate.ptr == NULL)
+    if (!priv->map_transfer)
 	return;
 
     exa_debug_printf("ExaFinishAccess %d\n", index);