asahi: Identify "Sample mask after depth/stencil" bit

Corresponds to Metal [[sample_mask,post_depth_coverage]].

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19811>

src/asahi/lib/cmdbuf.xml

@@ -394,6 +394,7 @@
     <field name="Unk 1" size="1" start="9" type="bool" default="true"/>
     <!-- Or discards? -->
     <field name="No colour output" size="1" start="21" type="bool"/>
+    <field name="Sample mask after depth/stencil" size="1" start="25" type="bool"/>
     <field name="Lines or points" size="1" start="26" type="bool"/>
     <field name="Pass type" size="3" start="29" type="Pass type"/>
   </struct>