asahi: cmdbuf: Fix scratch bucket offset/size

This is 4 bits. It is an opaque value that gets passed to the
helper, except 0 means no scratch.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27616>

src/asahi/lib/cmdbuf.xml

@@ -708,7 +708,7 @@
     <field name="Tag" size="8" start="0" type="USC Control" default="Registers"/>
     <field name="Register count" size="5" start="8" type="uint" modifier="groups(8)"/>
     <field name="Unk 1" size="1" start="13" type="bool" default="false"/>
-    <field name="Spill size" size="8" start="16" type="hex" default="0"/> <!-- TODO: determine relation, see docs/table.py -->
+    <field name="Spill size" size="4" start="18" type="hex" default="0"/>
     <field name="Unk 4" size="8" start="24" type="hex" default="0x1"/>
   </struct>