drm/hibmc: Drop drm_vblank_cleanup

So this seems to be the first driver that does it the right way round,
so fix it up by calling drm_atomic_helper_shutdown instead. We need to
do that before the last kms user is gone (fbdev emulation), but before
we start shutting down hw stuff like interrupts.

Cc: Xinliang Liu <z.liuxinliang@hisilicon.com>
Cc: Rongrong Zou <zourongrong@gmail.com>
Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Cc: Chen Feng <puck.chen@hisilicon.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-3-daniel.vetter@ffwll.ch