drm/i915: Warn if stealing non pipe A/B power sequencer

There's no power sequencer on pipe C on VLV/CHV so scream a bit if we
try to steal one from pipes other than A and B.

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

drivers/gpu/drm/i915/intel_dp.c

@@ -2691,6 +2691,9 @@ static void vlv_steal_power_sequencer(struct drm_device *dev,
 
 	lockdep_assert_held(&dev_priv->pps_mutex);
 
+	if (WARN_ON(pipe != PIPE_A && pipe != PIPE_B))
+		return;
+
 	list_for_each_entry(encoder, &dev->mode_config.encoder_list,
 			    base.head) {
 		struct intel_dp *intel_dp;