drm/i915/selftests: Always hold a reference on a waited upon request

Whenever we wait on a request, make sure we actually hold a reference to
it and that it cannot be retired/freed on another CPU!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191121071044.97798-4-chris@chris-wilson.co.uk