drm/i915/selftests: Check for any sign of request starting in wait_for_submit()

We only want to wait until the request has been submitted at least once;
that is it is either in flight, or has been.

References: fcf7df7a

 ("drm/i915/selftests: Check for the error interrupt before we wait!")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200218141305.1258394-1-chris@chris-wilson.co.uk