iommu/vt-d: Zero out allocated memory in dmar_enable_qi

kmemcheck complained about the use of uninitialized memory.
Fix by using kzalloc instead of kmalloc.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Joerg Roedel <joro@8bytes.org>