gma500: silence an unused variable warning

If CONFIG_BACKLIGHT_CLASS_DEVICE is disabled then GCC warns that:
	drivers/gpu/drm/gma500/opregion.c:154:6: warning:
		unused variable ‘max’ [-Wunused-variable]

Which give me a chance to use the new config_enabled() macro.  :)

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>