mesa/st: Use memset to zero out struct

mesa/st: Use memset to zero out struct

This is a non-stop source of warnings and build breakage. memset works
everywhere.

src/mesa/state_tracker/st_tgsi_lower_depth_clamp.c:354:45: warning:
suggest braces around initialization of subobject [-Wmissing-braces]

Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5174>