drm/i915: Avoid requesting a zero-sized stolen object

The stolen allocator objects loudly if the caller requests a zero-sized
object. This is a useful verbose check as in most cases the request
should have been pruned much early. Here we just want to silently return
before attempting the allocation.

Regression from
commit 484b41dd
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Mar 7 08:57:55 2014 -0800

    drm/i915: remove early fb allocation dependency on CONFIG_FB v2

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75963


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>