drm/i915/gvt: Mark i915.enable_gvt as false if loading fails

If we update the value of i915.enable_gvt should we fail to load GVT,
userspace can easily detect when it fails to load as requested.

Testcase: igt/gvt_basic
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466507234-23242-1-git-send-email-chris@chris-wilson.co.uk