Release playback PCM when there is no data
Release playback PCM when there is no data

Grabbing PCM activates underlying device. If there is no data (data
might not arrive for a long time), there is no need to keep device
in an active state, because this will prevent device to enter power
saving mode.

Fixes #129

Arkq/bluez-alsa,Arkq/bluez-alsa,Arkq/bluez-alsa