pan/mdg: Fix discard encoding

Let's match the blob.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6525>

src/panfrost/midgard/midgard_emit.c

@@ -643,7 +643,8 @@ emit_branch(midgard_instruction *ins,
         int quadword_offset = 0;
 
         if (is_discard) {
-                /* Ignored */
+                /* Fixed encoding, not actually an offset */
+                quadword_offset = 0x2;
         } else if (is_tilebuf_wait) {
                 quadword_offset = -1;
         } else if (target_number > block->base.name) {