DRI2: Don't waste overlay on a strange 1x1 window created by gnome-shell

We manage only a single hardware overlay. That's a precious shared
resource, which we want to use for zero-copy fullscreen compositing
in gnome-shell. The strange 1x1 window does not really need it.
Fixes https://github.com/ssvb/xf86-video-sunxifb/issues/2

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>