drm/xe/guc: Refactor GuC debugfs initialization

We don't have to drmm_kmalloc() local copy of debugfs_list to
write there our pointer to the struct xe_guc as we can extract
pointer to the struct xe_gt from the grandparent debugfs entry,
in similar way to what we did for GT debugfs files.

Note that there is no change in file/directory structure, just
refactored how files are created and how functions are called.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250403142635.1821-2-michal.wajdeczko@intel.com