drm/omap: Remove shadow lut usage

Removing extraneous shadow lut maintenance.  There is no need for
this to be in place until power management is added to the driver,
and this extra copy degrades performance for no gain.

Signed-off-by: Andy Gross <andy.gross@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>