Discard the empty video_set_lut function from platform video
drivers.

This commit "69d27545" introduces
a weak function video_set_lut, so we do not need an strong function
in platform drivers, which does nothing.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Reviewed-by: Tom Rini <trini@konsulko.com>