drm/i915/gvt: Don't mark vgpu context as inactive when preempted

We shouldn't mark inactive for vGPU context if preempted,
which would still be re-scheduled later. So keep active state.

Fixes: d6c05113 ("drm/i915/execlists: Distinguish the incomplete context notifies")
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>