OMAPDSS: HDMI: Split audio_enable into audio_enable/disable

To improve readability, split the audio_enable HDMI IP operation
into two separate functions for enabling and disabling audio.
The audio_enable function is also modified to return an error value.

While there, update these operations for the OMAP4 IP accordingly.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>