drm/nouveau/device: Get right pgsize_bitmap of iommu_domain

drm/nouveau/device: Get right pgsize_bitmap of iommu_domain

The supported page sizes of an iommu_domain are saved in the pgsize_bitmap
field. Retrieve the value from the right place.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20211218074546.1772553-1-baolu.lu@linux.intel.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20220216025249.3459465-7-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>