drm/sti: Don't call drm_helper_disable_unused_functions

Atomic drivers are supposed to do hw/sw state reset with the
drm_mode_config_reset() call right above it.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-38-git-send-email-daniel.vetter@ffwll.ch