i965/draw: Properly handle rounding when dividing by InstanceDivisor

The old code always divided rounded down and then subtracted 1.  What we
wanted was to divide rounded up and then subtract 1 which is equivalent to
subtracting 1 and then dividing rounded down.

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
1 file changed