iommu/vt-d: Fix a variable set but not used

The commit "iommu/vt-d: Delegate the dma domain to upper layer" left an
unused variable,

drivers/iommu/intel-iommu.c: In function 'disable_dmar_iommu':
drivers/iommu/intel-iommu.c:1652:23: warning: variable 'domain' set but
not used [-Wunused-but-set-variable]

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Joerg Roedel <jroedel@suse.de>