drm/i915/gvt: Avoid use-after-free iterating the gtt list

Found by smatch:

drivers/gpu/drm/i915/gvt/gtt.c:2452 intel_vgpu_destroy_ggtt_mm() error: dereferencing freed memory 'pos'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>