mesa: don't wait in _mesa_ClientWaitSync if timeout is 0
From ARB_sync spec:
If the value of <timeout> is zero, then ClientWaitSync does not
block, but simply tests the current state of <sync>. TIMEOUT_EXPIRED
will be returned in this case if <sync> is not signaled, even though
no actual wait was performed.
Fixes random fails of the arb_sync-timeout-zero piglit test on r600g.
Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit
b05a1fc156c4776d97d6ff3dcce71e6e34bac21d)