iommu: simplify list initialization in iommu_create_device_direct_mappings()

iommu: simplify list initialization in iommu_create_device_direct_mappings()

Use LIST_HEAD() to declare and initialize the 'mappings' list head in
iommu_create_device_direct_mappings() instead of separate declaration and
INIT_LIST_HEAD(). This simplifies the code by combining declaration and
initialization into a single idiomatic form, improving readability without
changing functionality.

Signed-off-by: Can Peng <pengcan@kylinos.cn>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>