drm/atomic: shutdown *current* encoder

In disable_outputs() we need to shut down the outgoing encoder, not the
incoming one (we have already swapped-state at this point).  Without
this, we end up telling the driver to crtc->dpms(OFF) without first
encoder->dpms(OFF), and that makes some hw quite unhappy.

v2: missing WARN_ON() hunk and comment
Reviewed-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -499,9 +499,12 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
 		if (!old_conn_state || !old_conn_state->crtc)
 			continue;
 
-		encoder = connector->state->best_encoder;
+		encoder = old_conn_state->best_encoder;
 
-		if (!encoder)
+		/* We shouldn't get this far if we didn't previously have
+		 * an encoder.. but WARN_ON() rather than explode.
+		 */
+		if (WARN_ON(!encoder))
 			continue;
 
 		funcs = encoder->helper_private;