D3D11: Simplify load function table init. Now that C++11 constructor syntax is allowed, we can use a much simpler scheme. BUG=angleproject:1407 BUG=chromium:616176 Change-Id: Ida5f235c3ed1429564af5213a5fead6c3b896080 Reviewed-on: https://chromium-review.googlesource.com/350903 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>