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

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

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>