drm/atomic-helper: Fix leak in disable_all

The legacy plane->fb pointer is refcounted by calling
drm_atomic_clean_old_fb().

In practice this isn't a real problem because:
- The caller in the i915 gpu reset code restores the original state
  again, which means the plane->fb pointer won't change, hence can't
  leak.
- Drivers using drm_atomic_helper_shutdown call the fbdev cleanup
  first, and that usually cleans up the fb through
  drm_remove_framebuffer, which does this correctly.
- Without fbdev the only framebuffers are from userspace, and those
  get cleaned up (again using drm_remove_framebuffer) befor the driver
  can even be unloaded.

But in i915 I've switched the cleanup sequence around so that the
_shutdown() calls happens after the drm_remove_framebuffer(), which is
how I discovered this issue.

v2: My analysis why the current code was ok for gpu reset and
suspend/resume was correct, but then I totally failed to realize that
we better keep this symmetric. Thanksfully CI noticed that for
balance, a refcounting bug must exist at 2 places if previously there
was no issue ...

v3: Don't be lazy and compute the plane_mask in
commit_duplicated_state properly too, instead of just using ~0U.

Cc: martin.peres@free.fr
Cc: chris@chris-wilson.co.uk
Acked-by: Dave Airlie <airlied@gmail.com> (v1)
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170715093106.19873-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_atomic_helper.c

@@ -2526,6 +2526,7 @@ int drm_atomic_helper_disable_all(struct drm_device *dev,
 	struct drm_plane *plane;
 	struct drm_crtc_state *crtc_state;
 	struct drm_crtc *crtc;
+	unsigned plane_mask = 0;
 	int ret, i;
 
 	state = drm_atomic_state_alloc(dev);
@@ -2568,10 +2569,14 @@ int drm_atomic_helper_disable_all(struct drm_device *dev,
 			goto free;
 
 		drm_atomic_set_fb_for_plane(plane_state, NULL);
+		plane_mask |= BIT(drm_plane_index(plane));
+		plane->old_fb = plane->fb;
 	}
 
 	ret = drm_atomic_commit(state);
 free:
+	if (plane_mask)
+		drm_atomic_clean_old_fb(dev, plane_mask, ret);
 	drm_atomic_state_put(state);
 	return ret;
 }
@@ -2702,11 +2707,16 @@ int drm_atomic_helper_commit_duplicated_state(struct drm_atomic_state *state,
 	struct drm_connector_state *new_conn_state;
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *new_crtc_state;
+	unsigned plane_mask = 0;
+	struct drm_device *dev = state->dev;
+	int ret;
 
 	state->acquire_ctx = ctx;
 
-	for_each_new_plane_in_state(state, plane, new_plane_state, i)
+	for_each_new_plane_in_state(state, plane, new_plane_state, i) {
+		plane_mask |= BIT(drm_plane_index(plane));
 		state->planes[i].old_state = plane->state;
+	}
 
 	for_each_new_crtc_in_state(state, crtc, new_crtc_state, i)
 		state->crtcs[i].old_state = crtc->state;
@@ -2714,7 +2724,11 @@ int drm_atomic_helper_commit_duplicated_state(struct drm_atomic_state *state,
 	for_each_new_connector_in_state(state, connector, new_conn_state, i)
 		state->connectors[i].old_state = connector->state;
 
-	return drm_atomic_commit(state);
+	ret = drm_atomic_commit(state);
+	if (plane_mask)
+		drm_atomic_clean_old_fb(dev, plane_mask, ret);
+
+	return ret;
 }
 EXPORT_SYMBOL(drm_atomic_helper_commit_duplicated_state);