iommu/dma: Remove redundant locking

This reverts commit ac9a5d52.

iommu_dma_init_domain() is now only called under the group mutex, as it
should be given that that the default domain belongs to the group, so
the additional internal locking is no longer needed.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/a943d4c198e6a1fffe998337d577dc3aa7f660a9.1740585469.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>