drm/i915/selftests: Pick correct caching mode.

Caching mode is HW dependent so pick a correct one using
intel_gt_coherent_map_type().

Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/10249


Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Acked-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-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/20240312111815.18083-1-nirmoy.das@intel.com