st/mesa: disallow deferred flush if there are multiple contexts

u_threaded can hang in these situation, with one context waiting on a deferred fence from the other context. But the other context isn't flushing its pending work (because it's waiting for more work to pushed) so everything is stuck.

Fixes: d17b35e6 ("gallium: add PIPE_FLUSH_DEFERRED")

Closes: #1430 (closed)

Edited by Pierre-Eric Pelloux-Prayer

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