x86_64: disable the GART in shutdown

For K8 system: 4G RAM with memory hole remapping enabled, or more than 4G
RAM installed.  when using kexec to load second kernel.  In the second
kernel, when mem is allocated for GART, it will do the memset for clear, it
will cause restart, because some device still used that for dma.  solution
will be:

in second kernel: disable that at first before we try to allocate mem for
it.  or in the first kernel: do disable that before shutdown.
Andi/Eric/Alan prefer to second one for clean shutdown in first kernel.
Andi also point out need to consider to AGP enable but mem less 4G case
too.

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Muli Ben-Yehuda <muli@il.ibm.com>
Cc: Vivek Goyal <vgoyal@in.ibm.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>