drm/xe/uapi: Document drm_xe_query_gt

Split drm_xe_query_gt out of the gt list one in order to better
document it.

No functional change at this point. Any actual change to the
uapi should come in follow-up additions.

v2: s/maks/mask

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