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: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56C2E686.5060803@linux.intel.comReviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>