ASoC: dapm - Make DAPM reset code a separate function.

It's useful to export the DAPM reset as a static function for future use
by other DAPM functions. e.g. The dynamic PCM query widgets resets the DAPM
graph before working out active paths.

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