nvc0/ir: always use CG mode for loads from atomic-only buffers

Atomic operations don't update the local cache, which means that we
would have to issue CCTL operations in order to get the updated values.
When we know that a buffer is primarily used for atomic operations, it's
easier to just avoid the caching at that level entirely.

The same issue persists for non-atomic buffers, which will have to be
fixed separately.

Fixes the failing dEQP-GLES31.functional.atomic_counter.* tests.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Cc: 19.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 4443b6dd)