iommu/dma: Fix for dereferencing before null checking

The cookie is dereferenced before null checking in the function
iommu_dma_init_domain.

This patch moves the dereferencing after the null checking.

Fixes: fdbe574e ("iommu/dma: Allow MSI-only cookies")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>