drm/rockchip: Drop drm_vblank_cleanup

Either not relevant (in the load error paths) or done better already
(in the unload code, by calling drm_atomic_helper_shutdown). Drop it.

Cc: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-9-daniel.vetter@ffwll.ch