drm/xe/pf: Check for fence error on VRAM save/restore

drm/xe/pf: Check for fence error on VRAM save/restore

The code incorrectly assumes that the VRAM save/restore fence is valid.
Fix it by checking for error.

Fixes: 49cf1b9b609fe ("drm/xe/pf: Handle VRAM migration data as part of PF control")
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20251114122339.1791026-1-michal.winiarski@intel.com
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>