iommu/vt-d: Clear the redundant assignment in dmar_enable_qi

__dmar_enable_qi() will initialize free_head,free_tail and
free_cnt for q_inval. Remove the redundant initialization
in dmar_enable_qi().

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>