It's not a core function, and the matching atomic functions are also
not in the core. Plus the suspend/resume helper is also already there.

Needs a tiny bit of open-coding, but less midlayer beats that I think.

v2: Rebase onto ast (which gained a new user).

Cc: Sam Bobroff <sbobroff@linux.ibm.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NSean Paul <sean@poorly.run>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: Rex Zhu <Rex.Zhu@amd.com>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Shaoyun Liu <Shaoyun.Liu@amd.com>
Cc: Monk Liu <Monk.Liu@amd.com>
Cc: nouveau@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20181217194303.14397-4-daniel.vetter@ffwll.ch