drm/i915: Check before removing mm notifier

Error in mmu_interval_notifier_insert() can leave a NULL
notifier.mm pointer. Catch that and return early.

Fixes: ed29c269 ("drm/i915: Fix userptr so we do not have to worry about obj->mm.lock, v7.")
Cc: <stable@vger.kernel.org> # v5.13+
[tursulin: Added Fixes and cc stable.]
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Shawn Lee <shawn.c.lee@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219125047.28906-1-nirmoy.das@intel.comSigned-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/gem/i915_gem_userptr.c

@@ -349,6 +349,9 @@ i915_gem_userptr_release(struct drm_i915_gem_object *obj)
 {
 	GEM_WARN_ON(obj->userptr.page_ref);
 
+	if (!obj->userptr.notifier.mm)
+		return;
+
 	mmu_interval_notifier_remove(&obj->userptr.notifier);
 	obj->userptr.notifier.mm = NULL;
 }