drm/xe/guc: Add yet another helper macro for threshold

We already have and use MAKE_GUC_KLV_VF_CFG_THRESHOLD_KEY, but
in upcoming patch we want to generate the key for the length.
Add MAKE_GUC_KLV_VF_CFG_THRESHOLD_LEN for this purpose.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240919171528.1451-2-michal.wajdeczko@intel.com