drm/sti: Remove dummy ->load_lut() implementation

The ->load_lut() callback is optional, therefore a dummy implementation
is not needed.

Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>