drm/i915: Move disable_cxsr to the crtc_state.

intel_crtc->atomic will be removed later on, move this member
to intel_crtc_state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447945645-32005-2-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>