drm: bridge/dw_hdmi: introduce interfaces to enable and disable audio

iMX6 devices suffer from an errata (ERR005174) where the audio FIFO can
be emptied while it is partially full, resulting in misalignment of the
audio samples.

To prevent this, the errata workaround recommends writing N as zero
until the audio FIFO has been loaded by DMA.  Writing N=0 prevents the
HDMI bridge from reading from the audio FIFO, effectively disabling
audio.

This means we need to provide the audio driver with a pair of functions
to enable/disable audio.  These are dw_hdmi_audio_enable() and
dw_hdmi_audio_disable().

A spinlock is introduced to ensure that setting the CTS/N values can't
race, ensuring that the audio driver calling the enable/disable
functions (which are called in an atomic context) can't race with a
modeset.

Tested-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>