drm/imx: Add active plane reconfiguration support

We don't support configuring active plane on-the-fly for imx-drm.
The relevant CRTC should be disabled before the plane configuration.
Of course, the plane itself should be disabled as well.
This patch adds active plane reconfiguration support by forcing CRTC
mode change and disabling-enabling plane in plane's ->atomic_update
callback.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Tested-by: Peter Senna Tschudin <peter.senna@gmail.com>
Tested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>