ASoC: Factor out boiler plate for DAPM event generation

Make the DAPM sequence execution look a bit nicer by factoring out the
code to invoke an event into a single function since it's all the same
pretty much.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>