ASoC: Add helper function getting CODEC's DAPM context

The DAPM context in the snd_soc_codec struct is redundant and scheduled to
be replaced by the DAPM context in the snd_soc_component struct. This patch
introduces a new helper function snd_soc_codec_get_dapm() which should be
used for getting the DAPM context for a CODEC rather then directly
accessing the dapm field. Once there are no more direct users of the dapm
field left it is possible to transparently switch all drivers to the
component DAPM context by updating snd_soc_codec_get_dapm() function.

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