drm/i915/gvt: add enable_execlists check before enable gvt

The GVT-g needs execlists to be enabled otherwise gvt should be
disabled. Add a check for enable_execlists before enabling gvt.

v2: use DRM_INFO in response to the user action

Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>