nak/sm50: Use MemScope::CTA for constant image loads

Using MemScope::System synchronizes with everything, which is exactly
what we don't want for constant loads.  This is currently a no-op
because we aren't using MemScope::Constant pre-Ampere yet.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35217>

src/nouveau/compiler/nak/sm50.rs

@@ -2454,8 +2454,7 @@ impl SM50Op for OpSuLd {
         // mem_eviction_policy not a thing for sm < 70
 
         let scope = match self.mem_order {
-            MemOrder::Constant => MemScope::System,
-            MemOrder::Weak => MemScope::CTA,
+            MemOrder::Constant | MemOrder::Weak => MemScope::CTA,
             MemOrder::Strong(s) => s,
         };