audiomixer: update to work with WirePlumber 0.5

audiomixer: update to work with WirePlumber 0.5

This update also brings improvements to the initialization state
management, so that ensure_controls() is guaranteed to return after
all the relevant controls have been discovered. Previously there were
cases that ensure_controls() would return without having discovered
the role-based sinks or the equalizer sink.

Bug-AGL: SPEC-4934
Change-Id: If3acca37c98ae6ff5ef811b7634951d12bf1d030
Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>