drm/xe: Prepare format for GT-oriented messages in one place

To avoid code duplication (and thus potential mistakes) and to
allow easier changes (if needed) of the prefix format of the
GT-oriented messages, prepare that prefix in dedicated macro.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250909165941.31730-4-michal.wajdeczko@intel.com