mixer-control: Force set default sink if mixer changes the output

Some audio output device may disappear if it is not connected, for
example the HDMI stereo of AMD Oland R7-430 graphic card. That makes the
mixer control's UI and PulseAudio's output state mismatch.

This patch always updates PulseAudio's state when mixer control changes
the output.

Fixes: GNOME/libgnome-volume-control#13
1 job for I13 in 3 minutes and 7 seconds
Status Name Job ID Coverage
passed build-fedora #727360