drm/i915/gvt: Don't submit request for error workload dispatch

As vGPU shadow ctx is loaded with guest context state, arbitrarily
submitting request in error workload dispatch path would cause trouble.
So don't try to submit in error path now like in previous code.
This is to fix VM failure when GPU hang happens.

Fixes: f0e99437 ("drm/i915/gvt: Fix workload request allocation before request add")
Reviewed-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>