ASoC: OMAP: HDMI: Move HDMI codec trigger function to generic HDMI driver

The function hdmi_audio_trigger is a callback used by ASoC to stop/start
HDMI audio. Also, it does not perform IP-specific configuration directly.
Hence, it should be placed in the general portion of the HDMI driver,
along with the other ASoC callbacks.

Also, make this function static.

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