drm/imx: ipuv3-plane: fix atomic update status query for non-plus i.MX6Q

The current buffer check halves the frame rate on non-plus i.MX6Q,
as the IDMAC current buffer pointer is not yet updated when
ipu_plane_atomic_update_pending is called from the EOF irq handler.

Fixes: 70e8a0c7

 ("drm/imx: ipuv3-plane: add function to query atomic update status")
Tested-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Cc: stable@vger.kernel.org