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

UPSTREAM: 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)

Change-Id: I58c5b794cdafa6c9f2594376fc2e98918156e409
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>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
(cherry picked from 1494276000db789c6d2acd85747be4707051c801)