drm/i915/selftests: Flush tasklet on wait_for_submit()

Always flush the tasklet if we have pending submissions in
wait_for_submit(), so that even if we see the HW has started before we
process its ack, when we return the execlists state is well defined.

Fixes: 06289949 ("drm/i915/selftests: Check for any sign of request starting in wait_for_submit()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200218211215.1336341-1-chris@chris-wilson.co.uk