drm/atomic-helper: Implement subsystem-level suspend/resume

Provide subsystem-level suspend and resume helpers that can be used to
implement suspend/resume on atomic mode-setting enabled drivers.

v2: simplify locking, enhance kerneldoc comments
v3: pass lock acquisition context by parameter, improve kerneldoc
v4: - remove redundant code (already provided by atomic helpers)
      (Maarten Lankhorst)
    - move backoff dance from drm_modeset_lock_all_ctx() into suspend
      helper (Daniel Vetter)
v5: handle potential EDEADLK from drm_atomic_helper_duplicate_state()
    and drm_atomic_helper_disable_all() (Daniel Vetter)

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449075005-13937-2-git-send-email-thierry.reding@gmail.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 6b4cf25..10d0989 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -855,6 +855,12 @@
  * due to slight differences in allocating shared resources when the
  * configuration is restored in a different order than when userspace set it up)
  * need to use their own restore logic.
+ *
+ * This function is deprecated. New drivers should implement atomic mode-
+ * setting and use the atomic suspend/resume helpers.
+ *
+ * See also:
+ * drm_atomic_helper_suspend(), drm_atomic_helper_resume()
  */
 void drm_helper_resume_force_mode(struct drm_device *dev)
 {