mesa: fix SignalSemaphoreEXT behavior

the EXT_external_object spec originally was underspecified with regards to this function, leaving room for synchronization errors where:

• app calls SignalSemaphoreEXT to signal a semaphore
• mesa defers pipe_context::fence_server_signal with threaded context
• driver defers gpu submission
• SignalSemaphoreEXT has long since returned, app submits vk cmdbuf waiting on semaphore
• spec violation / device lost

to prevent this, the spec is being changed to:

1. require an implicit flush when calling SignalSemaphoreEXT
2. require that this implicit flush also forces GPU submission before SignalSemaphoreEXT returns

all affected drivers have been updated

fixes #6568 (closed)

cc: mesa-stable