drm/i915/mtl: workaround coherency issue for Media

This patch implements Wa_22016122933.

In MTL, memory writes initiated by the Media tile update the whole
cache line, even for partial writes. This creates a coherency
problem for cacheable memory if both CPU and GPU are writing data
to different locations within a single cache line.
This patch circumvents the issue by making CPU/GPU shared memory
uncacheable (WC on CPU side, and PAT index 2 for GPU).  Additionally,
it ensures that CPU writes are visible to the GPU with an
intel_guc_write_barrier().

While fixing the CTB issue, we noticed some random GSC firmware
loading failure because the share buffers are cacheable (WB) on CPU
side but uncached on GPU side. To fix these issues we need to map
such shared buffers as WC on CPU side. Since such allocations are
not all done through GuC allocator, to avoid too many code changes,
the i915_coherent_map_type() is now hard coded to return WC for MTL.

v2: Simplify the commit message(Matt).

BSpec: 45101

Signed-off-by: Fei Yang <[email protected]>
Reviewed-by: Andi Shyti <[email protected]>
Acked-by: Nirmoy Das <[email protected]>
Reviewed-by: Andrzej Hajda <[email protected]>
Reviewed-by: Matt Roper <[email protected]>
Signed-off-by: Nirmoy Das <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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