drm/sti: Use drm_atomic_helper_shutdown

drm/sti: Use drm_atomic_helper_shutdown

drm_plane_helper_disable is a non-atomic drivers only function, and
will blow up (since no one passes the locking context it needs).

Atomic drivers which want to quiescent their hw on unload should
use drm_atomic_helper_shutdown() instead.

The sti cleanup code seems supremely confused:
- In the load error path it calls drm_mode_config_cleanup before it
  stops various kms services like poll worker or fbdev emulation.
  That's going to oops.
- The actual unload code doesn't even bother with the cleanup and just
  leaks.

Try to fix this while at it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-13-daniel.vetter@ffwll.ch