drm/i915/gt: Temporarily disable CPU caching into DMA for MTL

FIXME: It is suspected that some Address Translation Service (ATS)
issue on IOMMU is causing CAT errors to occur on some MTL workloads.
Applying a write barrier to the ppgtt set entry functions appeared
to have no effect, so we must temporarily use I915_MAP_WC in the
map_pt_dma class of functions on MTL until a proper ATS solution is
found.

Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
CC: Chris Wilson <chris.p.wilson@linux.intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Acked-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231102175831.872763-1-jonathan.cavitt@intel.com