drm/i915: Allow ggtt lookups to not WARN

To be able to effectively use the GGTT object lookup function, we don't
want to warn when there is no GGTT mapping. Let the caller deal with it
instead.

Originally, I had intended to have this behavior, and has not
introduced the WARN. It was introduced during review with the addition
of the follow commit

commit 5c2abbea
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Tue Sep 24 09:57:57 2013 -0700

    drm/i915: Provide a cheap ggtt vma lookup

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>