drm/i915: save AUD_FREQ_CNTRL state at audio domain suspend

When audio power domain is suspended, the display driver must
save state of AUD_FREQ_CNTRL on Tiger Lake and Ice Lake
systems. The initial value of the register is set by BIOS and
is read by driver during the audio component init sequence.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920083918.27057-1-kai.vehmanen@linux.intel.com