drm/i915/gvt: Refine port select logic for CFL platform

Refine the code since the port select definition for CFL is different
than SKL/BXT.

v2:
- replace PCH_CNP with IS_COFFEELAKE. (zhenyu)

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