drm: bridge/dw_hdmi: protect n/cts setting with a mutex

The HDMI n/cts settings need to be updated whenever the audio sample
rate or the video pixel clock changes.  This needs to be protected
against concurrency as there is no synchronisation between these two
operations.  Introduce a mutex (called audio_mutex) to protect against
two threads trying to update the video clock rate and pixel clock
simultaneously.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>