iommu/vt-d: Correctly disable Intel IOMMU force on

I made a mistake in commit bfd20f1c. We should skip the force on with the
option enabled instead of vice versa. Not sure why this passed our
performance test, sorry.

Fixes: bfd20f1c ('x86, iommu/vt-d: Add an option to disable Intel IOMMU force on')
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>