drm/xe: Don't keep stale pointer to bo->ggtt_node

When we fail to map a BO in the GGTT, we release our GGTT node
placeholder, but leave stale bo->ggtt_node pointer to it, which
triggers an assert immediately followed by a crash, due to UAF:

[ ] xe 0000:00:02.0: [drm] Assertion `bo->ggtt_node->base.size == bo->size` failed!
[ ] WARNING: CPU: 4 PID: 126 at drivers/gpu/drm/xe/xe_ggtt.c:689 xe_ggtt_remove_bo+0x1d9/0x250 [xe]
[ ] RIP: 0010:xe_ggtt_remove_bo+0x1d9/0x250 [xe]
[ ] Call Trace:
[ ]  <TASK>
[ ]  ? __warn+0x88/0x190
[ ]  ? xe_ggtt_remove_bo+0x1d9/0x250 [xe]
[ ]  ? report_bug+0x1c3/0x1d0
[ ]  ? handle_bug+0x42/0x70
[ ]  ? exc_invalid_op+0x14/0x70
[ ]  ? asm_exc_invalid_op+0x16/0x20
[ ]  ? xe_ggtt_remove_bo+0x1d9/0x250 [xe]
[ ]  ? xe_ggtt_remove_bo+0x1d9/0x250 [xe]
[ ]  xe_ttm_bo_destroy+0x11f/0x260 [xe]
[ ]  ? ttm_bo_release+0x31c/0x350 [ttm]
[ ]  ? __mutex_unlock_slowpath+0x35/0x270
[ ]  __xe_bo_create_locked+0x4a0/0x550 [xe]
[ ]  ? mark_held_locks+0x49/0x80
[ ]  xe_bo_create_pin_map_at+0x37/0x200 [xe]
[ ]  xe_bo_create_pin_map+0x11/0x20 [xe]

While around, for similar reason, also don't keep an error pointer
if we fail to allocate ggtt_node placeholder.

Fixes: 34e80422 ("drm/xe: Make xe_ggtt_node struct independent")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240906220348.1836-1-michal.wajdeczko@intel.com