drm: delay vblank cleanup until after driver unload

Drivers may use vblank calls now (e.g. drm_vblank_off) in their unload
paths, so don't clean up the vblank related structures until after
driver unload.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>