It's silly to have 2 mallocs when we could tie these two together.

Also, Gustavo adds another one in his per-crtc out-fence patches. And
I want to add more stuff here for nonblocking commit helpers.

In the future we can use this to store a pointer to the preceeding
state, making an atomic update entirely free-standing. This will be
needed to be able to queue them up with a depth > 1.

Cc: Gustavo Padovan <gustavo@padovan.org>
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/1464818821-5736-12-git-send-email-daniel.vetter@ffwll.ch