drm/xe/uapi: Replace useless 'instance' per unique gt_id

Let's have a single GT ID per GT within the PCI Device Card.

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