We can read out the pipe HW state only if the required power domain is
on. If not we consider the pipe to be off.

v2:
- no change
v3:
- push down the power domain checks into the specific crtc
  get_pipe_config handlers (Daniel)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Appease checkpatch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>