drm/i915/selftests: Check for an initial-breadcrumb in wait_for_submit()

When we look at i915_request_is_started() we must be careful in case we
are using a request that does not have the initial-breadcrumb and
instead the is-started is being compared against the end of the previous
request. This will make wait_for_submit() declare that a request has
been already submitted too early.

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/20200519063123.20673-4-chris@chris-wilson.co.uk