intel: make sure we update the renderbuffers after a swap

Now that LOCK_HARDWARE is gone, we don't have a convenient place to
update the renderbuffers everywhere we need them.  So grab new buffers
when we invalidate the old ones until we optimize things further.

src/mesa/drivers/dri/intel/intel_screen.c

@@ -132,6 +132,8 @@ intelDRI2FlushInvalidate(__DRIdrawable *drawable)
 
    intelDRI2Flush(drawable);
    drawable->validBuffers = GL_FALSE;
+
+   intel_update_renderbuffers(intel->driContext, drawable);
 }
 
 static const struct __DRI2flushExtensionRec intelFlushExtension = {