radeonsi: use CP DMA for the null const buffer clear on CIK

This is a workaround for a thread deadlock that I have no idea
why it occurs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108879


Fixes: 9b331e46

Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit b58e5fb6)