mesa: don't wait in _mesa_ClientWaitSync if timeout is 0

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>