frontends/va: fix potential overflows

The multiplication of 32 bits integers will be truncated before
being widened to the destination variable' size.
Reported by static analysis.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35877>
1 file changed