drm/i915/gt: Prevent queuing retire workers on the virtual engine

Virtual engines are fleeting. They carry a reference count and may be freed
when their last request is retired. This makes them unsuitable for the
task of housing engine->retire.work so assert that it is not used.

Tvrtko tracked down an instance where we did indeed violate this rule.
In virtual_submit_request, we flush a completed request directly with
__i915_request_submit and this causes us to queue that request on the
veng's breadcrumb list and signal it. Leading us down a path where we
should not attach the retire.

Reported-by: Tvrtko Ursulin <[email protected]>
Fixes: dc93c9b69315 ("drm/i915/gt: Schedule request retirement when signaler idles")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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