drm/i915: Always flush the active worker before returning from the wait

The first thing the active retirement worker does is decrement the
i915_active count.

The first thing we do during i915_active_wait is try to increment the
i915_active count, but only if already active [non-zero].

The wait may see that the retirement is already started and so marked the
i915_active as idle, and skip waiting for the retirement handler.
However, the caller of i915_active_wait may immediately free the
i915_active upon returning (e.g. i915_vma_destroy) so we must not return
before the concurrent access from the worker is completed. We must
always flush the worker.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2473
Fixes: 274cbf20fd10 ("drm/i915: Push the i915_active.retire into a worker")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: <[email protected]> # v5.5+
Reviewed-by: Matthew Auld <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit 977a372e972cb42799746c284035a33c64ebace9)
Signed-off-by: Jani Nikula <[email protected]>

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