drm/xe: Store pointer to struct xe_gt in gt/ debugfs directory

drm/xe: Store pointer to struct xe_gt in gt/ debugfs directory

Attributes added under 'gt/' directories may wish to use that
in case they can't obtain it from elsewhere.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20240214115756.1525-2-michal.wajdeczko@intel.com
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328162808.451-2-michal.wajdeczko@intel.com