drm/i915/selftests: Be paranoid and flush the tasklet before checking status

When waiting for the submit, before checking the status of the request,
kick the tasklet to make sure we are processing the submission. This
speeds up submission if we are using any tasklet suppression for
secondary requests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222113536.3775-3-chris@chris-wilson.co.uk