drm/xe: Do not dereference ggtt_node in xe_bo.c

drm/xe: Do not dereference ggtt_node in xe_bo.c

A careful inspection of __xe_ggtt_insert_bo_at() shows that
the ggtt_node can always be seen as inserted from xe_bo.c
due to the way error handling is performed.

The checks are also a little bit too paranoid, since we
never create a bo with ggtt_node[id] initialised but not
inserted into the GGTT, which can be seen by looking at
__xe_ggtt_insert_bo_at()

Additionally, the size of the GGTT is never bigger than 4 GB,
so adding a check at that level is incorrect.

Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260108101014.579906-14-dev@lankhorst.se