x86, dmar: check if it's initialized before disable queue invalidation

If queue invalidation is disabled after it's already initialized,
dmar_enable_qi won't re-enable it due to iommu->qi is allocated.
It may result in system hang when use queue invalidation. Add this
check to avoid this case.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>