drm/i915: Don't use BAR mappings for ring buffers with LLC

Direction from hardware is that ring buffers should never be mapped
via the BAR on systems with LLC. There are too many caching pitfalls
due to the way BAR accesses are routed. So it is safest to just not
use it.

Signed-off-by: John Harrison <[email protected]>
Fixes: 9d80841ea4c9 ("drm/i915: Allow ringbuffers to be bound anywhere")
Cc: Chris Wilson <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v4.9+
Tested-by: Jouni Högander <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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