[media] omap3isp: stat: Store sg table in ispstat_buffer

The driver stores the IOMMU mapped iovm struct pointer in the buffer
structure but only needs the iovm sg table. Store the sg table instead
to prepare the migration to the DMA API.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>