Only iterate the enabled draw buffers, and the depth/stencil buffer.

This series of small optimizations gives about a 15% improvement on
the draw call benchmark for the D3D11 backend with the null driver.

BUG=angleproject:1155

Change-Id: I5b29362f93c016c146d2a6527b378853bc070239
Reviewed-on: https://chromium-review.googlesource.com/666046Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>