iommu/amd: Hide unused iommu_table_lock

The newly introduced lock is only used when CONFIG_IRQ_REMAP is enabled:

drivers/iommu/amd_iommu.c:86:24: error: 'iommu_table_lock' defined but not used [-Werror=unused-variable]
 static DEFINE_SPINLOCK(iommu_table_lock);

This moves the definition next to the user, within the #ifdef protected
section of the file.

Fixes: ea6166f4 ("iommu/amd: Split irq_lookup_table out of the amd_iommu_devtable_lock")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>