drm/xe: Make xe_ggtt_node_insert return a node

drm/xe: Make xe_ggtt_node_insert return a node

This extra step is easier to handle inside xe_ggtt.c and makes
xe_ggtt_node_allocated a simple null check instead, as the intermediate
state 'allocated but not inserted' is no longer used.

Privatize xe_ggtt_node_fini() and init() as they're no longer used
outside of xe_ggtt.c

Reviewed-by: Matthew Brost <matthew.brost@intel.com> #v1
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20260206112108.1453809-11-dev@lankhorst.se
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>