drm/i915: take a power domain reference while checking the HDMI live status

drm/i915: take a power domain reference while checking the HDMI live status

There are platforms that don't need the full GMBUS power domain (BXT)
while others do (PCH, VLV/CHV). For optimizing this we would need to add
a new power domain, but it's not clear how much we would benefit given
the short time we hold the reference. So for now let's keep things
simple.

This fixes a regression introduced in
commit 237ed86c693d8a8e4db476976aeb30df4deac74b
Author: Sonika Jindal <sonika.jindal@intel.com>
Date:   Tue Sep 15 09:44:20 2015 +0530

   drm/i915: Check live status before reading edid

v2:
- fix commit message, PCH won't take any redundant power resource after
this change (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[fix commit message in v2 (Imre)]
[Cherry-picked from drm-intel-next-queued 29bb94bb (Imre)]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448643329-18675-6-git-send-email-imre.deak@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>