ASoC: Intel: Skylake: Remove redundant vmixer handler

Initially vmixer and mixer widget handlers were bit different, but over
time they became same so remove the duplicate code.

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>