drm/i915: Fix DMA mapped scatterlist walks

When walking DMA mapped scatterlists sg_dma_len has to be used since it
can be different (coalesced) from the backing store entry.

This also means we have to end the walk when encountering a zero length
DMA entry and cannot rely on the normal sg list end marker.

Both issues were there in theory for some time but were hidden by the fact
Intel IOMMU driver was never coalescing entries. As there are ongoing
efforts to change this we need to start handling it.

v2:
 * Use unsigned int for local storing sg_dma_len. (Logan)

Signed-off-by: Tvrtko Ursulin <[email protected]>
References: 85d1225ec066 ("drm/i915: Introduce & use new lightweight SGL iterators")
References: b31144c0daa8 ("drm/i915: Micro-optimise gen6_ppgtt_insert_entries()")
Reported-by: Tom Murphy <[email protected]>
Suggested-by: Tom Murphy <[email protected]> # __sgt_iter
Suggested-by: Logan Gunthorpe <[email protected]> # __sgt_iter
Cc: Joonas Lahtinen <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Lu Baolu <[email protected]>
Reviewed-by: Logan Gunthorpe <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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