drm/xe/pf: Don't advertise support to enable VFs if not ready

Even if we have not enabled SR-IOV support using the platform
specific has_sriov flag, the hardware may still report SR-IOV
capability and the PCI layer may wrongly advertise driver support
to enable VFs.  Explicitly reset the number of supported VFs to
zero to avoid confusion.

Applications may read the /sys/bus/pci/devices/.../sriov_totalvfs
prior to enabling VFs using the sriov_numvfs to check if such an
operation is possible.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240507165757.2835-1-michal.wajdeczko@intel.com