iommu/amd: Set variable amd_dirty_ops to static

iommu/amd: Set variable amd_dirty_ops to static

Fix the followng warning:
drivers/iommu/amd/iommu.c:67:30: warning: symbol
 'amd_dirty_ops' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20231120095342.1102999-1-chentao@kylinos.cn
Signed-off-by: Joerg Roedel <jroedel@suse.de>