x86: turn off iommu merge by default

revert this commit for now:

    commit 9480626
    Author: Andi Kleen <ak@suse.de>
    Date:   Fri Oct 19 20:35:03 2007 +0200

        x86: enable iommu_merge by default

it's causing regressions:

    http://bugzilla.kernel.org/show_bug.cgi?id=9412

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/pci-dma_64.c

@@ -12,7 +12,7 @@
 #include <asm/gart.h>
 #include <asm/calgary.h>
 
-int iommu_merge __read_mostly = 1;
+int iommu_merge __read_mostly = 0;
 EXPORT_SYMBOL(iommu_merge);
 
 dma_addr_t bad_dma_address __read_mostly;