iris: Only set key->flat_shade if COL0/COL1 are written.

This was just laziness on my part, we already added similar checks in
the VS key handling.  Just need to do it here too.  Should improve cache
hits.

src/gallium/drivers/iris/iris_context.h

@@ -475,6 +475,7 @@ struct iris_vtable {
    void (*populate_gs_key)(const struct iris_context *ice,
                            struct brw_gs_prog_key *key);
    void (*populate_fs_key)(const struct iris_context *ice,
+                           const struct shader_info *info,
                            struct brw_wm_prog_key *key);
    void (*populate_cs_key)(const struct iris_context *ice,
                            struct brw_cs_prog_key *key);