x86/iommu: Shrink the critical section in dmar_qi_task()

It is safe to test and clear the Invalidation Wait Descriptor
Complete flag before acquiring the DMAR lock in dmar_qi_task(),
rather than waiting until the lock is held.

Reviewed by: kib
MFC after: 2 weeks

(cherry picked from commit 4eaaacc75535befdb9894cca4e0d8da376328fa4)