drm/sun4i: Drop drm_vblank_cleanup

Again seems just cargo-culted ... It's not ordered against any
irq/vblank/modeset shutdown.

Cc: Maxime Ripard  <maxime.ripard@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-33-daniel.vetter@ffwll.ch