drm/i915: unify S3 and S4 suspend/resume handlers

The S3 and S4 events are now handled the same way internally, there is no
need to keep separate wrapper functions around them. Simply reuse the
suspend/resume versions everywhere.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>