drm/i915: Unbind closed vma for i915_gem_object_unbind()

Closed vma are removed from the obj->vma_list so that they cannot be
found by userspace. However, this means that when forcibly unbinding an
object, we have to wait upon all rendering to that object first in order
for the closed, but active, vma to be reaped and their bindings removed.

Reported-by: Matthew Auld <matthew.auld@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97343
Fixes: aa653a68 ("drm/i915: Be more careful when unbinding vma")
Fixes: 8a3b3d57

 (" drm/i915: Convert non-blocking userptr waits...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471196681-30043-2-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Tested-by: Matthew Auld <matthew.auld@intel.com>