I've fumbled this in
commit 
f02ad907cd9e7fe3a6405d2d005840912f1ed258
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jan 22 16:36:23 2015 +0100
    drm/atomic-helpers: Recover full cursor plane behaviour
and accidentally put the assignment for legacy_cursor_upate after the
atomic commit, where it is pretty useless.
Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
        plane_state->src_h = src_h;
        plane_state->src_w = src_w;
 
+       if (plane == crtc->cursor)
+               state->legacy_cursor_update = true;
+
        ret = drm_atomic_commit(state);
        if (ret != 0)
                goto fail;
 
-       if (plane == crtc->cursor)
-               state->legacy_cursor_update = true;
-
        /* Driver takes ownership of state on successful commit. */
        return 0;
 fail: