drm/xe: Move TLB invalidation variable to own sub-structure in GT

TLB invalidations no longer just restricted to USM, move the variables
to own sub-structure.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>