agx: Condition writeout ops on already being emitted

There's a lot of r/e waiting here, but will be needed for ld_tile. Match
the Metal blob.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10720>

src/asahi/compiler/agx_compiler.h

@@ -318,6 +318,9 @@ typedef struct {
    /* For creating temporaries */
    unsigned alloc;
 
+   /* I don't really understand how writeout ops work yet */
+   bool did_writeout;
+
    /* Stats for shader-db */
    unsigned loop_count;
    unsigned spills;