gma500: Sort out dither

Ubuntu users reported that dithering was not being set on Poulsbo, and they
have a point as we set one variable and check another which is never set.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Tested-by: Luca Forina <luca.forina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>