freedreno/a6xx: pre-calculate expected vsc stream sizes

freedreno/a6xx: pre-calculate expected vsc stream sizes

We should only rely on overflow detection for indirect draws, where we
have no other option.

This doesn't use quite the worst-possible-case sizes, which in practice
seem to be ~20x larger than what is required.  But instead uses roughly
half of that.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4750>