drm/exynos: enable hdmi audio feature

This patch adds hdmi audio feature for exynos drm.
With this patch, i2s channel feeds audio data in hdmi when hdmi is connected.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>