freedreno/a6xx: don't emit a bogus size for empty cb slots

Noticed that asphalt9 had no uniforms bound, so cb[0] is null.  In
theory shouldn't cause a problem, since nothing is doing `ldc` against
cb[0], but to be safe we should use SIZE=0.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6117>
1 file changed