drm/i915: remove dead code from legacy suspend handler

The legacy DRM suspend logic (effective in UMS) doesn't handle any S4 thaw
events so we don't need to care about it either. Only S3 suspend and S4
freeze events are handled. Leave an assert behind to be sure.

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>