Framebuffer: Fix enabled draw buffers bitset. This would start off the zeroth element as enabled even if there was no attachment. If we remove the extra set, then we no longer have to check if the attachment is valid when iterating over the enabled bitset. BUG=angleproject:2107 Change-Id: I679f66156ced09d42add9e302ac974166713e0d5 Reviewed-on: https://chromium-review.googlesource.com/581884 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>