drm/i915/selftests: Set always_coherent to false when reading from CPU

Commit 8d4ba9fc ("drm/i915/selftests: Pick correct caching mode.")
was not complete  as for non LLC  sharing platforms cpu read can happen
from LLC which probably doesn't have the latest changes made by GPU.

Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Fixes: 8d4ba9fc ("drm/i915/selftests: Pick correct caching mode.")
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240516151403.2875-1-nirmoy.das@intel.com


Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
(cherry picked from commit 007ed70831426d4cc108d879d688de6b8e3e6d45)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>