drm/xe: Move ggtt_fini to devm managed

ggtt->scratch is destroyed via devm, ggtt_fini sets ggtt->scratch to
NULL, ggtt->scratch in GGTT clears, so ensure ggtt->scratch is set NULL
before the BO is destroyed.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240820172958.1095143-2-matthew.brost@intel.com