drm/i915/gvt: Fix GPU hang after reusing vGPU instance across different guest OS

We have implemented delayed ring mmio switch mechanism to reduce
unnecessary mmio switch. While the vGPU is being destroyed or
detached from VM, we need to force the ring switch to host context.

The later deadline is missed. Then it got a chance that word load
from VM2 might execute under the ring context of VM1 which was
attached to a same vGPU instance. Finally, the GPU is hang.

This patch guarantee the two deadline are performed.

v2: Remove unused variable 'scheduler'

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>