OMAPDSS: HDMI: Create function to enable HDMI audio

In order to separate clearly IP-specific code from general DSS code,
a function for OMAP4 audio enable is created. This function is
included in the HDMI IP ops to align with the current implementation
of the DSS HDMI driver. This function is to be used by the ASoC
HDMI audio codec.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>