ASoC: dapm: Add helpers to lock/unlock DAPM mutex

Acquiring the DAPM mutex is necessary before using several DAPM
functions and dereference is quite ugly. This patch provides a helper
function to simplify this.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>