EXT_draw_buffers may not be enabled but the maxColorAttachments cap is
always initialized so make sure to validate for the extension instead of
just checking that the attachment is in the valid range.

BUG=angleproject:2058

Change-Id: I5b48cb496bf96cbc0911295aa5bf87784ce9241b
Reviewed-on: https://chromium-review.googlesource.com/735749Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>