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.

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

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[fix commit message in v2 (Imre)]
Link: http://patchwork.freedesktop.org/patch/msgid/1447959301-1263-2-git-send-email-imre.deak@intel.com