drm/xe/vf: Don't emit access to Global HWSP if VF

VFs can't access Global HWSP, don't emit questionable MI_FLUSH_DW
while processing a migration job.

Bspec: 52398
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240405133936.891-2-michal.wajdeczko@intel.com