drm/i915/gvt: Off by one in intel_vgpu_write_fence()

The > should be >= here so that we don't read one element beyond the
end of the array.

Fixes: 28a60dee

 ("drm/i915/gvt: vGPU HW resource management")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>