drm: Update plane->fb also for page_flip

The legacy page_flip driver entry point is the only one left which
requires drivers to update plane->fb themselves. All the other entry
hooks will patch things up for the driver as needed since no one seems
to reliable get this right, see e.g. drm_mode_set_config_internal or
the plane->fb/old_fb handling in drm_mode_atomic_ioctl.

Therefore unify things, which allows us to ditch a TODO from
drm_atomic_helper_page_flip.

This should also help the atomic transition in i915 since we keep a
bit of legacy cruft only around because of this special behaviour in
->page_flip.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -1915,10 +1915,6 @@ retry:
 	if (ret != 0)
 		goto fail;
 
-	/* TODO: ->page_flip is the only driver callback where the core
-	 * doesn't update plane->fb. For now patch it up here. */
-	plane->fb = plane->state->fb;
-
 	/* Driver takes ownership of state on successful async commit. */
 	return 0;
 fail:

drivers/gpu/drm/drm_crtc.c

@@ -5345,13 +5345,7 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 		/* Keep the old fb, don't unref it. */
 		crtc->primary->old_fb = NULL;
 	} else {
-		/*
+		crtc->primary->fb = fb;
-		 * Warn if the driver hasn't properly updated the crtc->fb
-		 * field to reflect that the new framebuffer is now used.
-		 * Failing to do so will screw with the reference counting
-		 * on framebuffers.
-		 */
-		WARN_ON(crtc->primary->fb != fb);
 		/* Unref only the old framebuffer. */
 		fb = NULL;
 	}