drm/i915/ddi: Ensure the HW is powered during HW state readout

drm/i915/ddi: Ensure the HW is powered during HW state readout

The assumption when adding the intel_display_power_is_enabled() checks
was that if it returns success the power can't be turned off afterwards
during the HW access, which is guaranteed by modeset locks. This isn't
always true, so make sure we hold a dedicated reference for the time of
the access.

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455296121-4742-8-git-send-email-imre.deak@intel.com
(cherry picked from commit e27daab49718e3232318d8b539cb302521b4b724)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>