drm/xe/vf: Assume fixed GSM size if VF

VFs can't use size mirrored from PCI config, but it should be
safe to assume it covers full 4GiB GGTT.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240213154355.1221-2-michal.wajdeczko@intel.com