radeonsi: fix a nasty bug in si_pm4.c

If you did:
  si_pm4_set_reg(pm4, reg, val0);
  si_pm4_cmd_add(pm4, val1);
  si_pm4 set_reg(pm4, reg + 4, val1);

it wrote val0 to reg, val1 to reg + 4, and val2 to reg + 8.

This fixes it by clearing last_opcode in si_pm4_cmd_add, so that
si_pm4_set_reg doesn't try to combine set_reg calls across si_pm4_cmd_add.

Fixes: da78d50bc87 - radeonsi: make si_pm4_cmd_begin/end static and simplify all usages

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7721>
(cherry picked from commit 0d4f1dcd155be753ff49cd763be916cf701cda25)
diff --git a/.pick_status.json b/.pick_status.json
index 3f79954..f45ec88 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -733,7 +733,7 @@
         "description": "radeonsi: fix a nasty bug in si_pm4.c",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "master_sha": null,
         "because_sha": "da78d50bc87ef5db846a942664094b6299cd1888"
     },