Premultiply and color key control registers are already set to zero by
initialization code few lines above. Furthermore, it seems that
colorkeying doesn't really work. It's not used in BSP driver and
experiments with it all failed.

Just remove the code.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-6-jernej.skrabec@siol.net