drm/xe: Add helper to format SR-IOV function name

While the GuC firmware and the Xe driver are using VF identifier
VFID(0) to represent the Physical Function, we should avoid using
"VF0" name and use proper "PF" name in all user facing messages
related to the Physical Function and use "VFn" name only when
referrinf to the true Virtual Function. Add simple helper to get
properly formatted function name based on the function number.

Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240415173937.1287-2-michal.wajdeczko@intel.com