drm/i915/gt: Move user_forcewake application to GT

We already track the debugfs user_forcewake on the GT, so it is natural
to pull the suspend/resume handling under gt/ as well.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101141009.15581-3-chris@chris-wilson.co.uk