drm/i915/gvt: properly access enabled intel_engine_cs

Switch to use new for_each_engine() helper to properly access
enabled intel_engine_cs as i915 core has changed that to be
dynamic managed. At GVT-g init time would still depend on ring
mask to determine engine list as it's earlier.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>