x86: turn off iommu merge by default

revert this commit for now:

    commit 94806268
    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>