drm/i915: Fix audio power up sequence for gen10+ display

On platfroms with gen10+ display, driver must set the enable bit of
AUDIO_PIN_BUF_CTL register before transactions with the HDA controller
can proceed. Add setting this bit to the audio power up sequence.

Failing to do this resulted in errors during display audio codec probe,
and failures during resume from suspend.

Note: We may also need to disable the bit afterwards, but there are
still unresolved issues with that.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111214


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/20191003085531.30990-1-kai.vehmanen@linux.intel.com