dmabuf: Fix the disjointness check

We were checking the wrong fds here.

This came up while toying with udmabuf and creating multi-plane
NV12 buffers.

gdk/gdkdmabuf.c

@@ -2475,7 +2475,7 @@ gdk_dmabuf_is_disjoint (const GdkDmabuf *dmabuf)
     {
       struct stat plane_stat;
 
-      if (fstat (dmabuf->planes[0].fd, &plane_stat) != 0)
+      if (fstat (dmabuf->planes[i].fd, &plane_stat) != 0)
         return TRUE;
 
       if (first_stat.st_ino != plane_stat.st_ino)