frontend/va/image: add pipe flush for vlVaPutImage

To fix synchronization issue between multimedia queue and gfx queue.
Adding flush call will let multimedia queue to wait for the content of gfx
command buffer to be executed, for the case where there is dependency
between these two queues.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <mesa/mesa!8567>