drm/xe/vf: Add proper detection of the SR-IOV VF mode

SR-IOV VF mode detection is based on testing VF capability bit on
the register that is accessible from both the PF and enabled VFs.

Bspec: 49904, 53227
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240327182740.407-4-michal.wajdeczko@intel.com