drm/zte: Drop drm_vblank_cleanup

It again looks all cargo-culted for no good reasons.

Cc: Shawn Guo <shawnguo@kernel.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-13-daniel.vetter@ffwll.ch