drm/i915: take a power domain ref only when needed during HDMI detect

Suggested by Ville.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447959301-1263-1-git-send-email-imre.deak@intel.com