drm/sti: Drop drm_vblank_cleanup

Seems entirely cargo-culted.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-31-daniel.vetter@ffwll.ch