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>