drm/i915/execlists: Skip nested spinlock for validating pending

Only along the submission path can we guarantee that the locked request
is indeed from a foreign engine, and so the nesting of engine/rq is
permissible. On the submission tasklet (process_csb()), we may find
ourselves competing with the normal nesting of rq/engine, invalidating
our nesting. As we only use the spinlock for debug purposes, skip the
debug if we cannot acquire the spinlock for safe validation - catching
99% of the bugs is better than causing a hard lockup.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Fixes: c95d31c3df1b ("drm/i915/execlists: Lock the request while validating it during promotion")
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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