Since commit 63c5d5c6c4, the surfaceless
platform has allowed creation of pbuffer surfaces. But the vtable entry
for eglSwapBuffers has remained NULL.
Discovered by running a little pbuffer test.
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>