asahi: be explicit about usc reg unk

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31908>

src/asahi/genxml/cmdbuf.xml

@@ -693,7 +693,7 @@
     <field name="Register count" size="5" start="8" type="uint" modifier="groups(8)"/>
     <field name="Unk 1" size="1" start="13" type="bool"/>
     <field name="Spill size" size="4" start="18" type="hex"/>
-    <field name="Unk 4" size="8" start="24" type="hex" default="0x1"/>
+    <field name="Unk 4" size="8" start="24" type="hex"/>
   </struct>
 
   <struct name="USC No Preshader" size="2">

src/asahi/lib/agx_linker.c

@@ -193,6 +193,7 @@ agx_fast_link(struct agx_linked_shader *linked, struct agx_device *dev,
       cfg.register_count = nr_gprs;
       cfg.unk_1 = fragment;
       cfg.spill_size = scratch_size ? agx_scratch_get_bucket(scratch_size) : 0;
+      cfg.unk_4 = 1;
    }
 
    if (fragment) {