The FB helper core now supports deferred setup, so the driver's custom
implementation can be removed.

v2: Dont' resurrect drm_vblank_cleanup.

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>
Signed-off-by: Thierry Reding <treding@nvidia.com> (v1)
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170706130023.28417-5-daniel.vetter@ffwll.chSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>