drm/atomic-helper: Really recover pre-atomic plane/cursor behavior

I've fumbled this in

commit f02ad907
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>