drm/xe: prevent potential UAF in pf_provision_vf_ggtt()

The node ptr can point to an already freed ptr, if we hit the path with
an already allocated node. We later dereference that pointer with:

	xe_gt_assert(gt, !xe_ggtt_node_allocated(node));

which is a potential UAF. Fix this by not stashing the ptr for node.
Also since it is likely a bad idea to leave config->ggtt_region pointing
to a stale ptr, also set that to NULL by calling
pf_release_vf_config_ggtt() instead of pf_release_ggtt().

Fixes: 34e80422 ("drm/xe: Make xe_ggtt_node struct independent")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240828104341.180111-2-matthew.auld@intel.com