GVT-g relies on the enable_execlists parameter in i915. If this option
is not enabled for GVT-g, should return -EIO to make i915 driver loading
failed.

v2:
- Use DMR_ERROR as it is a fatal message. (Chris)
Suggested-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: NChuanxiao Dong <chuanxiao.dong@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>