drm/xe/vf: Don't try to read legacy GuC MMIO notification if VF

Legacy SOFT_SCRATCH registers are not accessible from the VF. Any
G2H notification posted there will be handled by the PF driver.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240405133936.891-4-michal.wajdeczko@intel.com