drm/xe/uapi: Rename gts to gt_list

During the uapi review it was identified a possible confusion
with the plural of acronym with a new acronym. So the
recommendation is to go with gt_list instead.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>