drm/xe: Remove unused functions

xe_gt_topology_dss_group_mask and xe_gt_topology_count_dss are probably
leftover from initial implementation - they are not called from
anywhere. Remove those functions.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>