drm/i915/gvt: Return -EIO if host GuC submission is enabled when loading GVT-g

Currently GVT-g cannot work properly when host GuC submission is
enabled, so make the driver loading failed in this case.

v2:
- use DRM_ERROR as it is a fatal message. (Chris)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chuanxiao Dong <chuanxiao.dong@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1495878259-7733-2-git-send-email-chuanxiao.dong@intel.com