drm/i915/gvt: cancel virtual vblank timer when no vGPU exists

Stop irq timer for virtual vblank timer emulation if no vGPU exists,
otherwise it will keep gvt service thread busy to handle virtual vblank
but no use.

Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>