radeonsi: don't set READ_ONLY for const_uploader to fix bindless texture hangs

Bindless textures can update descriptors with WRITE_DATA.

Cc: 19.1 <mesa-stable@lists.freedesktop.org>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Acked-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 5058d62b)