drm/i915/gt: Cancel the flush worker more thoroughly

Since the worker may rearm, we currently are only guaranteed to flush
the work if we cancel the timer. If the work was running at the time we
try and cancel it, we will wait for it to complete, but it may leave
items in the pool and requeue the work. If we rearrange the immediate
discard of the pool then cancel the work, we know that the work cannot
rearm and so our flush will be final.

<0> [314.146044] i915_mod-1321    2.... 299799443us : intel_gt_fini_buffer_pool: intel_gt_fini_buffer_pool:227 GEM_BUG_ON(!list_empty(&pool->cache_list[n]))

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1920
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525141957.3061-1-chris@chris-wilson.co.uk

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