nak: Surface handles are not allowed to be rZ

The chances of this happening are near zero with the way we do surface
ops today but I have seen it in the wild and this is apparently a rule.
The hardware throws an illegal instruction encoding if it sees 255.

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35895>

src/nouveau/compiler/nak/ir.rs

@@ -5382,7 +5382,7 @@ pub struct OpSuLd {
     pub mem_order: MemOrder,
     pub mem_eviction_priority: MemEvictionPriority,
 
-    #[src_type(GPR)]
+    #[src_type(SSA)]
     pub handle: Src,
 
     #[src_type(SSA)]
@@ -5413,7 +5413,7 @@ pub struct OpSuSt {
     pub mem_order: MemOrder,
     pub mem_eviction_priority: MemEvictionPriority,
 
-    #[src_type(GPR)]
+    #[src_type(SSA)]
     pub handle: Src,
 
     #[src_type(SSA)]
@@ -5454,7 +5454,7 @@ pub struct OpSuAtom {
     pub mem_order: MemOrder,
     pub mem_eviction_priority: MemEvictionPriority,
 
-    #[src_type(GPR)]
+    #[src_type(SSA)]
     pub handle: Src,
 
     #[src_type(SSA)]