iommu/dma: Remove redundant locking

iommu/dma: Remove redundant locking

This reverts commit ac9a5d522bb80be50ea84965699e1c8257d745ce.

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>