Added implementation for case target_sbc == 0. In that case, the
function shall schedule a wait until all pending swaps for the drawable
have completed.
Fix for non-blocking case. Old implementation returned random,
uninitialized values for (ust,msc,sbc) if it returned immediately
without scheduling a wait due to sbc >= target_sbc.
Now if function doesn't schedule a wait, but returns immediately,
it returns the (ust,msc,sbc) of the most recently completed swap,
i.e., the UST and MSC corresponding to the time when the returned
current SBC was reached.
Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
751e8c09d3