dri: Fix driWaitForMSC32 when divisor >= 2 and msc < 0.

dri: Fix driWaitForMSC32 when divisor >= 2 and msc < 0.

We'd come up with a negative remainder, while we were looking for the positive
version of it in the loop conditional.  And, since the "did we hit our target"
break was disabled for the target_msc == 0 ("Just make the divisor/remainder
work") path, we'd never exit.

Simplify the code by just using int64_t all over instead of trying to do it
in a u32 space.