drm/i915: send proper opregion notifications on suspend/resume

This indicates to the firmware that it can power down various other
components or bring them back up, depending on the target system state.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>