ASoC: dapm: Only mark paths dirty when the connection status changed

Rework soc_dapm_{mixer,mux}_update_power() to only mark a path dirty if the
connect state if the path has actually changed. This avoids unnecessary
power state checks for the widgets involved.

Also factor out the common code that is involved in this into a helper
function.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>