drm/imx: ipuv3-plane: Switch EBA buffer 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
switch EBA buffer(for hardware double buffering) in ipu_plane_atomic_set_base()
or not is no more correct.  Instead, we should do that only when we don't need
modeset, otherwise, we initialize the two EBA buffers with the buffer address.

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>