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

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

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>