drm/atomic-helper: remove modeset_lock_all from helper_resume

Atomic code rely shouldn't rely on the magic hidden acquire context.

v2: Remove unused config local var (gcc).

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/20170403083304.9083-5-daniel.vetter@ffwll.ch