OMAPDSS: HDMI: Implement DSS driver interface for audio

Implement the DSS device driver audio support interface in the HDMI
panel driver and generic driver. The implementation relies on the
IP-specific functions that are defined at DSS probe time.

A mixed locking strategy is used. The panel's mutex is used when
the state of the panel is queried as required by the audio functions.
The audio state is protected using a spinlock as users of DSS HDMI
audio functionality might start/stop audio while holding a spinlock.
The mutex and the spinlock are held and released as needed by each
individual function to protect the panel state and the audio state.

Although the panel's audio_start functions does not check whether
the panel is active, the audio _ENABLED state can be reached only
from audio_enable, which does check the state of the panel. Also,
if the panel is ever disabled, the audio state will transition
to _DISABLED. Transitions are always protected by the audio lock.

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