drm/i915: Remove defunct intel_suspend_gt_powersave()

Since commit b7137e0c ("drm/i915: Defer enabling rc6 til after we
submit the first batch/context"), intel_suspend_gt_powersave() has been
a no-op. As we still do not need to do anything explicitly on suspend
(we do everything required on idling), remove the defunct function.

References: b7137e0c

 ("drm/i915: Defer enabling rc6 til after we submit the first batch/context")
Suggested-by: "Hiatt, Don" <don.hiatt@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190323214009.23294-1-chris@chris-wilson.co.uk