radv: don't even attempt to prefetch on SI

radv: don't even attempt to prefetch on SI

Before bcae327469 this was emitting CP DMA packet even on SI, but
apparently hasn't caused too many problems. After that commit the
CP DMA code now always sets the CIK+ only bit for prefetch. Just
follow radeonsi there and don't try to prefetch at all.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101334
Fixes: bcae327469 "radv: realign cp dma code with radeonsi"
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>