x64, x2apic/intr-remap: use CONFIG_DMAR for DMA-remapping specific code

DMA remapping specific code covered with CONFIG_DMAR in
the generic code which will also be used later for enabling Interrupt-remapping.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: akpm@linux-foundation.org
Cc: arjan@linux.intel.com
Cc: andi@firstfloor.org
Cc: ebiederm@xmission.com
Cc: jbarnes@virtuousgeek.org
Cc: steiner@sgi.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>