drm/i915: Use CRTC index consistently during getting/putting CRTC power domains

The for_each_oldnew_intel_crtc_in_state() iterator index does match
crtc->pipe, but using the same thing as array index when getting and
putting CRTC power domains makes things clearer.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130212200.2811939-2-imre.deak@intel.com