drm/xe/vf: Don't initialize stolen memory manager if VF

VF drivers don't have access to the stolen memory.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240213154355.1221-5-michal.wajdeczko@intel.com