media: saa7146: use sg_dma_len when building pgtable

The new AMD IOMMU DMA implementation concatenates sglist entries under
certain conditions, and because saa7146 accessed the length member
directly, it did not support this scenario.

This fixes IO_PAGE_FAULTs by using the sg_dma_len macro.

Fixes: be62dbf5 ("iommu/amd: Convert AMD iommu driver to the dma-iommu api")
Signed-off-by: Tasos Sahanidis <tasos@tasossah.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>