drm/vgem: switch to postclose

I didn't spot anything that would require ordering here (well not
anywhere else either), and I'm trying to unify at least modern drivers
on one close hook.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-18-daniel.vetter@ffwll.ch