TGL supports more than three iDisp DAI's.
Add support for iDisp4 CPU DAI.

Without this patch, we saw the below error on our TGL DUT:
   sof_rt5682 tgl_max98357a_rt5682: ASoC: CPU DAI iDisp4 Pin not registered
Signed-off-by: Sathyanarayana Nujella <sathyanarayana.nujella@intel.com>
Signed-off-by: Jairaj Arava <jairaj.arava@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191220171037.10689-2-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>