drm/meson: Drop drm_vblank_cleanup

Again seems just cargo-culted.

Cc: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-26-daniel.vetter@ffwll.ch