drm/xe: Adjust mmio code to pass VF substructure to SRIOV code

drm/xe: Adjust mmio code to pass VF substructure to SRIOV code

Although we want to break the GT-centric nature of the MMIO code in the
general driver, the SRIOV handling still relies on data in a VF
substructure of the GT.  So add a GT backpointer, but name it
sriov_vf_gt to make it clear that it's only for this one specific
special case and will not be set or usable for anything else.

v2:
 - Store backpointer to the GT itself rather than the SRIOV-specific
   substructure.  (Michal)

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>  # v1
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240910234719.3335472-53-matthew.d.roper@intel.com