drm/xe: Annotate default for guc_log_level param

Reword the parameter description so it's clear what's the default and
what are the verbose levels.

Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://lore.kernel.org/r/20250613-guc-log-level-v2-2-cb84a63e49fe@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>