drm/xe/configfs: Always return consistent max_vfs value

drm/xe/configfs: Always return consistent max_vfs value

The max_vfs parameter used by the Xe driver has its default value
definition, but it could be altered by the module parameter or by
the device specific configfs attribute.

To avoid mistakes or code duplication, always rely on the configfs
helper (or stub), which will provide necessary fallback if needed.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/20260121214218.2817-4-michal.wajdeczko@intel.com