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>