drm/i915/gvt: Stop checking for impossible interrupts from a kthread

The kthread will not be interrupted, don't even bother checking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>