freedreno/ir3: Avoid {0} initializer for struct reginfo

First element is not a scalar.  Just initialize the struct like we do
elsewhere.

src/freedreno/ir3/disasm-a3xx.c:958:33: warning: suggest braces around
initialization of subobject [-Wmissing-braces]

Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5174>