drm/stm: Drop drm_vblank_cleanup

Again seems just cargo-culted.

Cc: Yannick Fertre <yannick.fertre@st.com>
Cc: Philippe Cornu <philippe.cornu@st.com>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-32-daniel.vetter@ffwll.ch