drm/i915: Use for_each_power_domain() in i915_power_domain_info()

Macro seems to do exactly the same thing.

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1486559530-15141-1-git-send-email-joonas.lahtinen@linux.intel.com