intel/gma: consolidate vbt code

Change-Id: I80b7facfb9cc9f642dd1c766884dc23da1aab2c8
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6800
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>