ASoC: hdac_hdmi: introduce macro to access HDMI private data

This patch replaces the direct access of HDMI private data with macro
hdev_to_hdmi_priv in order to prepare the code to remove hdac_ext_device
usage in the subsequent patch.
Signed-off-by: Rakesh Ughreja <rakesh.a.ughreja@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>