drm/atomic: Update old_fb after setting a property.
This change updates the old_fb pointer only after acquiring the plane lock,
if there are no properties the fb cannot have been changed either, so
this works out correctly.
Found in a discussion with Rob Clark.
Cc: Rob Clark <robdclark@gmail.com> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Reviewed-by: Rob Clark <robdclark@gmail.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>