drm/xe: Define Virtual Function Identifier

According to the PCI Express specification, the SR-IOV Virtual
Functions (VFs) are numbered starting with 1 (VF1, VF2, ...).
Additionally, both driver and GuC will refer to Physical Function
(PF) as VF0. Define helper macro to represent VFn and PF.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20240104222031.277-3-michal.wajdeczko@intel.com


Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>