anv: Flush the queue on DeviceWaitIdle

anv: Flush the queue on DeviceWaitIdle

When we have syncobj_wait, we can trust in WAIT_FOR_SUBMIT but when we
don't, we only have BO waits and those aren't quite as nice.  This
commit adds a flag to _anv_queue_submit to wait for the queue to drain
before returning.  This gives us the behavior we need to implement
DeviceWaitIdle.

Fixes: 246261f0add "anv: prepare the driver for delayed submissions"
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>