drm/i915: Move GGTT fence registers under gt/

Since the fence registers control HW detiling through the GGTT
aperture, make them a part of the intel_ggtt under gt/

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/20200316113846.4974-1-chris@chris-wilson.co.uk