drm/i915: fix up error cleanup in i915_gem_object_bind_to_gtt

This has been broken in

commit 2f633156
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Jul 17 12:19:03 2013 -0700

    drm/i915: Create VMAs

which resulted in an OOPS the first time around we've hit -ENOSPC.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67156


Cc: Imre Deak <imre.deak@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Tested-by: meng <mengmeng.meng@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>