ASoC: Export dapm_kcontrol_get_value

The DSP driver needs to know widget control value in its event handler for
widgets like mixers. This is required in the subsequent patches

Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>