drm/i915: Fix up documentation paths after file moving

Redirect references to i915_gem_fence_reg.c to gt/intel_ggtt_fencing.c

Fixes: dec9cf9e ("drm/i915/gt: Pull restoration of GGTT fences underneath the 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/20200317141250.20903-1-chris@chris-wilson.co.uk