drm/imx: ipuv3-plane: Skip setting u/vbo only when we don't need modeset

We added active plane reconfiguration support by forcing a full modeset
operation.  So, looking at old_plane_state->fb to determine whether we need
to set u/v offset(aka, u/vbo for IPUv3) in ipu_plane_atomic_set_base()
or not is no more correct.  Instead, we should do that only when we don't
need modeset.

Fixes: c6c1f9bc ("drm/imx: Add active plane reconfiguration support")
Cc: stable@vger.kernel.org # 4.8
Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>