drm/i915: remove duplicate definition of for_each_power_domain

drm/i915: remove duplicate definition of for_each_power_domain

commit fed899875444413699b31d23c27ab7f71a586660
Author: Borun Fu <borun.fu@intel.com>
Date:   Sat Jul 12 10:02:27 2014 +0530

    drm/i915: Power gating display wells during i915_pm_suspend

moved for_each_power_domain from intel_display.c to i915_drv.h but we
still have the definition around in intel_display.c, due to a merge
conflict resolution gone wrong in

commit f78cb674728ef2a5c7d8aaad262749ea4d4085c3
Merge: 88231098acc1 80c08879796a
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jul 29 20:49:36 2014 +0200

    Merge remote-tracking branch 'airlied/drm-next' into drm-intel-next

Just remove the extra definition left behind.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448383645-7615-1-git-send-email-jani.nikula@intel.com