drm/i915: Use atomic helpers for suspend, v2.

Instead of duplicating the functionality now that we no longer need
to preserve dpll state we can move to using the upstream suspend helper.

Changes since v1:
- Call hw readout with all mutexes held.
- Rework intel_display_suspend to only assign modeset_restore_state
  on success.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56C2E686.5060803@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 20d9dbd..6644c2e 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1848,6 +1848,7 @@
 
 	enum modeset_restore modeset_restore;
 	struct mutex modeset_restore_lock;
+	struct drm_atomic_state *modeset_restore_state;
 
 	struct list_head vm_list; /* Global list of all address spaces */
 	struct i915_gtt gtt; /* VM representing the global address space */