drm/atomic-helper: Add atomic_disable CRTC helper callback

Some display controllers need plane(s) to be disabled together with
the relevant CRTC, e.g., the IPUv3 display controller for imx-drm.
This patch adds atomic_disable CRTC helper callback so that
old_crtc_state(as a parameter of the callback) could be used
to get the active plane(s) of the old CRTC state for disable operation.

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: Peter Senna Tschudin <peter.senna@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1472196644-30563-2-git-send-email-gnuiyl@gmail.com