drm/mtk: Drop drm_vblank_cleanup

Seems entirely cargo-culted.

Cc: CK Hu <ck.hu@mediatek.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-6-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-7-daniel.vetter@ffwll.ch