drm/i915: grab the audio power domain when enabling audio on HSW+

drm/i915: grab the audio power domain when enabling audio on HSW+

With the current code, we unconditionally touch
HSW_AUD_PIN_ELD_CP_VLD, which means we can touch it when the power
well is off, and that will trigger an "Unclaimed register" message.

Just adding the intel_crtc->config.has_audio should already avoid the
unclaimed register messsages, but since we actually need the power
well to make the Audio code work, it makes sense to also grab the
audio power domain reference, and release it when it's not needed
anymore.

I used IGT's pm_rpm to reproduce this bug, but it can probably be
reproduced on other tests that do modesets. I'm using a machine with
eDP+HDMI connected.

Regression introduced by:

commit 23522c83619af613eb6093a5143d77f4d2d6489c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Apr 24 23:54:51 2014 +0200
    drm/i915: Simplify audio handling on DDI ports

Credits to Daniel for suggesting this implementation.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>