drm/i915: Fix oopses in the overlay code due to i915_gem_active stuff

The i915_gem_active stuff doesn't like a NULL ->retire hook, but
the overlay code can set it to NULL. That obviously ends up oopsing.
Fix it by introducing a new helper to assign the retirement callback
that will switch out the NULL function pointer with
i915_gem_retire_noop.

Cc: [e-mail]
Cc: Chris Wilson <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Fixes: 0d9bdd886f29 ("drm/i915: Convert intel_overlay to request tracking")
Signed-off-by: Ville Syrjälä <[e-mail]>
Signed-off-by: Chris Wilson <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit ecd9caa0522db5a6b03ac8858c42067ef9d8323b)
Signed-off-by: Jani Nikula <[e-mail]>

drivers/gpu/drm/i915/i915_gem_request.h

···
 	rcu_assign_pointer(active->request, request);
 }
 
+/**
+ * i915_gem_active_set_retire_fn - updates the retirement callback
+ * @active - the active tracker
+ * @fn - the routine called when the request is retired
+ * @mutex - struct_mutex used to guard retirements
+ *
+ * i915_gem_active_set_retire_fn() updates the function pointer that
+ * is called when the final request associated with the @active tracker
+ * is retired.
+ */
+static inline void
+i915_gem_active_set_retire_fn(struct i915_gem_active *active,
+			      i915_gem_retire_fn fn,
+			      struct mutex *mutex)
+{
+	lockdep_assert_held(mutex);
+	active->retire = fn ?: i915_gem_retire_noop;
+}
+
 static inline struct drm_i915_gem_request *
 __i915_gem_active_peek(const struct i915_gem_active *active)
 {

drivers/gpu/drm/i915/intel_overlay.c

···
 {
 	GEM_BUG_ON(i915_gem_active_peek(&overlay->last_flip,
 					&overlay->i915->drm.struct_mutex));
-	overlay->last_flip.retire = retire;
+	i915_gem_active_set_retire_fn(&overlay->last_flip, retire,
+				      &overlay->i915->drm.struct_mutex);
 	i915_gem_active_set(&overlay->last_flip, req);
 	i915_add_request(req);
 }