drm/i915/execlists: Flush pending preemption events during reset

drm/i915/execlists: Flush pending preemption events during reset

Catch up with the inflight CSB events, after disabling the tasklet
before deciding which request was truly guilty of hanging the GPU.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Michał Winiarski <[email protected]>
CC: Michel Thierry <[email protected]>
Cc: Jeff McGee <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]