drm/i915/gvt: check workload empty before real scan

This patch will fix warning log print during command scan caused by
empty workload (ring head equals tail). This patch avoid going into
real scan process if workload is empty. It's guest's responsibility
to make sure if an empty workload is proper to submit to HW.

[v2] modify the patch description. It's a fix, not a w/a.

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>