drm/i915/gvt: cancel scheduler timer when no vGPU exists

Stop gvt scheduler timer if no vGPU exists, otherwise it keeps
gvt service thread busy to handle request schedule event but no
actual schedule activity required.

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>