drm/i915: mark requests for GuC virtual engines to avoid use-after-free

In the Linux kernel, the following vulnerability has been resolved:

drm/i915: mark requests for GuC virtual engines to avoid use-after-free

References to i915_requests may be trapped by userspace inside a sync_file or dmabuf (dma-resv) and held indefinitely across different proceses. To counter-act the memory leaks, we try to not to keep references from the request past their completion. On the other side on fence release we need to know if rq->engine is valid and points to hw engine (true for non-virtual requests). To make it possible extra bit has been added to rq->execution_mask, for marking virtual engines.

(cherry picked from commit 280410677af763f3871b93e794a199cfcf6fb580)