iommu/amd: Use DEFINE_SPINLOCK() for spinlock

Spinlock can be initialized automatically with DEFINE_SPINLOCK()
rather than explicitly calling spin_lock_init().

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Link: https://lore.kernel.org/r/20201228135112.28621-1-zhengyongjun3@huawei.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>