drm/i915/gvt: Fix unnecessary schedule timer when no vGPU exits

From commit f25a49ab ("drm/i915/gvt: Use vgpu_lock to protect per
vgpu access") the vgpu idr destroy is moved later than vgpu resource
destroy, then it would fail to stop timer for schedule policy clean
which to check vgpu idr for any left vGPU. So this trys to destroy
vgpu idr earlier.

Cc: Colin Xu <colin.xu@intel.com>
Fixes: f25a49ab ("drm/i915/gvt: Use vgpu_lock to protect per vgpu access")
Acked-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200229055445.31481-1-zhenyuw@linux.intel.com