ASoC: dapm: lock mixer & mux update power with DAPM mutex

Both snd_soc_dapm_mux_update_power() and snd_soc_dapm_mixer_update_power() can
be called internally within DAPM core (with DAPM mutex held) and externally.

Provide some wrappers so that external users of both functions do not have to
remember to hold the DAPM mutex.

Signed-off-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>