drm/rcar-du: Use for_each_*_in_state

We want to hide drm_atomic_state internals better.

v2: Use drm_crtc_mask (Maarten).

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-5-git-send-email-daniel.vetter@ffwll.ch