gm45: Ensure that brightness register in gma contains sane value.

Change-Id: Ia66c71c3adf2ae0d413750b5e59e3eaba3888a0b
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6587
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>