dri glx: Swap before checking for cliprects.

We don't update drawables anymore unless they are completely
uninitialized, so we need to swap even if we don't have
cliprects yet, otherwise we never end up calling the driver's
SwapBuffers(). The driver should update the drawable in its
SwapBuffers() anyway.

See 8e753d04045a82062ac34d3b2622eb9dba8af374,
"dri glx: Fix dri_util::driBindContext" for the change that
exposed it.

src/mesa/drivers/dri/common/dri_util.c

@@ -311,12 +311,12 @@ static void driSwapBuffers(__DRIdrawable *dPriv)
     __DRIscreen *psp = dPriv->driScreenPriv;
     drm_clip_rect_t *rects;
     int i;
-    
-    if (!dPriv->numClipRects)
-        return;
 
     psp->DriverAPI.SwapBuffers(dPriv);
 
+    if (!dPriv->numClipRects)
+        return;
+
     rects = _mesa_malloc(sizeof(*rects) * dPriv->numClipRects);
 
     if (!rects)