drm/i915: Update dma-attributes for our sg DMA

Looking through the attributes for DMA mappings, it appears that by
default dma_map_sg will try and create a kernel accessible map of the
page. We never access this, as we either have a struct page already or
an iomap, so we can request that the dma mapper does not create one.
Without a kernel map in place, one presumes the rest of the memory
control attributes do not apply. We also explicitly control the caches
around the mappings, so we can ask it not to bother synchronising itself.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200706224308.22636-1-chris@chris-wilson.co.uk