drm/i915/gvt: Enable gfx virtualiztion for CFL

Use INTEL_GEN to simplify the code for SKL+ platforms.

v2:
- split the enabling code into final one to identify any regression.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Fei Jiang <fei.jiang@intel.com>
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>