It's acceptable to have the same object attached to muliple framebuffer
attachments if they are not being drawn to.

This was triggering failures in the conformance2/state/gl-object-get-calls.html
test.

BUG=483282

Change-Id: If088ccd9b1189f060dfa33ebbe4d82f1f47559b0
Reviewed-on: https://chromium-review.googlesource.com/354570Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>