{output,mixer}/alsa: use snd_pcm_poll_descriptors_revents()

This call was missing, causing very high CPU usage when the ALSA
output plugin was used with dmix.

Closes #391

src/mixer/plugins/AlsaMixerPlugin.cxx

@@ -117,6 +117,8 @@ AlsaMixerMonitor::DispatchSockets() noexcept
 {
 	assert(mixer != nullptr);
 
+	non_block.DispatchSockets(*this, mixer);
+
 	int err = snd_mixer_handle_events(mixer);
 	if (err < 0) {
 		FormatError(alsa_mixer_domain,