drm/i915: Cleanup gt powerstate from gem

Since the gt powerstate is allocated by i915_gem_init, clean it from
i915_gem_fini for symmetry and to correct the imbalance on error.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180812223642.24865-1-chris@chris-wilson.co.uk