drm/vc4: fix fb references in async update

commit c16b85559dcfb5a348cc085a7b4c75ed49b05e2c upstream.

Async update callbacks are expected to set the old_fb in the new_state
so prepare/cleanup framebuffers are balanced.

Calling drm_atomic_set_fb_for_plane() (which gets a reference of the new
fb and put the old fb) is not required, as it's taken care by
drm_mode_cursor_universal() when calling drm_atomic_helper_update_plane().

Cc: <stable@vger.kernel.org> # v4.19+
Fixes: 539c320bfa97 ("drm/vc4: update cursors asynchronously through atomic")
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603165610.24614-5-helen.koike@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/vc4/vc4_plane.c

@@ -818,6 +818,7 @@ static void vc4_plane_atomic_async_update(struct drm_plane *plane,
 		drm_atomic_set_fb_for_plane(plane->state, state->fb);
 	}
 
+	swap(plane->state->fb, state->fb);
 	/* Set the cursor's position on the screen.  This is the
 	 * expected change from the drm_mode_cursor_universal()
 	 * helper.