radeonsi: make si_fence_server_signal flush pipe without work
glSignalSemaphoreEXT sometime doesn't signal the semaphore, it is because radeonsi doesn't flush if gl context doesn't have pending work. Fix the porblem by always flush pipe.
glSignalSemaphoreEXT sometime doesn't signal the semaphore, it is because radeonsi doesn't flush if gl context doesn't have pending work. Fix the porblem by always flush pipe.