ASoC: topology: Consolidate how dtexts and dvalues are freed

Provide helper functions and use them to free dtexts and dvalues in
topology. This is followup cleanup after related changes in this area as
suggested in:
https://mailman.alsa-project.org/pipermail/alsa-devel/2019-January/144761.html



Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>