By blocking the attachment at this point, it doesn't allow for the resource to
have a depth stencil format later.

BUG=997702

Change-Id: Iec5243012cb9a9527c5b1467d44c393b0dc6bddc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1780898
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>