Turn on the DART already at 1GB. This is needed because of crippled
devices in some systems, i.e. Airport Extreme cards, only supporting
30-bit DMA addresses.

Otherwise, users with between 1 and 2GB of memory will need to manually
enable it with iommu=force, and that's no good.

Some simple performance tests show that there's a slight impact of
enabling DART, but it's in the 1-3% range (kernel build with disk I/O
as well as over NFS).

iommu=off can still be used for those who don't want to deal with the
overhead (and don't need it for any devices).

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>