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

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: d17b35e671a ("gallium: add PIPE_FLUSH_DEFERRED")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1430
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4213>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4213>