drm/imx: ipuv3-plane: Access old u/vbo properly in ->atomic_check for YU12/YV12

drm/imx: ipuv3-plane: Access old u/vbo properly in ->atomic_check for YU12/YV12

Before accessing the u/v offset(aka, u/vbo for IPUv3) of the old plane state's
relevant fb, we should make sure the fb is in YU12 or YV12 pixel format(which
are the two YUV pixel formats we support only), otherwise, we are likely to
trigger BUG_ON() in drm_plane_state_to_u/vbo() since the fb's pixel format is
probably not YU12 or YV12.

Link: https://bugs.freedesktop.org/show_bug.cgi?id=98150
Fixes: c6c1f9bc798b ("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>