drm/omap: Allow HDMI audio setup even if we do not have video configured

Allow HDMI audio setup even if we do not have video configured. Audio
will get configured at the same time with video if the video is
configured soon enough. If it is not the audio DMA will timeout in
couple of seconds and audio playback will be aborted.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>