drm/imx: disable planes before DC

If the DC clock is disabled before the attached IDMACs are properly
stopped the IDMACs may hang the IPU or even the whole system.

Make sure the IDMACs are in safe state by disabling the planes before
removal of the DC clock.

Also set the atomic parameter to false to stop calling the atomic_begin
hook, which does nothing useful as we immediately afterwards turn off
vblank interrupts and possibly send the pending vblank event.

Fixes: 33f14235

 (drm/imx: atomic phase 1: Use transitional atomic
                     CRTC and plane helpers)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>