suspend-vs-iommu: prevent suspend if we could not resume

iommu/gart support misses suspend/resume code, which can do bad stuff,
including memory corruption on resume.  Prevent system suspend in case we
would be unable to resume.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Tested-by: Patrick <ragamuffin@datacomm.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>