drm/i915: Initialize return value for empty i915_gem_object_unbind()

If the obj->vma_list is empty, we immediately return ret. However, we
are doing so having never set it to any value, it should be zero!

Reported-by: Matthew Auld <matthew.auld@intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=97343
Fixes: aa653a68

 ("drm/i915: Be more careful when unbinding vma")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471196681-30043-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>