zink: mark depth-component cube-maps as done

This worked fine all along, nothing to be done for this feature. The
piglit tests mostly pass.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5044>
diff --git a/docs/features.txt b/docs/features.txt
index e70d1cf..057adf1 100644
--- a/docs/features.txt
+++ b/docs/features.txt
@@ -61,7 +61,7 @@
   glGetStringi command                                  DONE
   glTexParameterI, glGetTexParameterI commands          DONE
   glVertexAttribI commands                              DONE
-  Depth format cube textures                            DONE ()
+  Depth format cube textures                            DONE (zink)
   GLX_ARB_create_context (GLX 1.4 is required)          DONE (v3d, zink)
   Multisample anti-aliasing                             DONE (freedreno/a5xx+, freedreno (*), llvmpipe (*), softpipe (*), swr (*), zink)
   8 draw buffers                                        DONE (zink)