mesa: update validation when draw buffer changes

Otherwise validation that depends on the _IntegerDrawBuffers and
_FP32DrawBuffers bitfield can end up stale.

Fixes: d04d9da98c ("st/mesa: fix _IntegerBuffers bitfield use")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35063>
(cherry picked from commit 3ec68e8382)

.pick_status.json

@@ -4954,7 +4954,7 @@
         "description": "mesa: update validation when draw buffer changes",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "d04d9da98c9f70ec7db59750558f10dad215b2f4",
         "notes": null

src/mesa/main/buffers.c

@@ -33,6 +33,7 @@
 #include "util/glheader.h"
 #include "buffers.h"
 #include "context.h"
+#include "draw_validate.h"
 #include "enums.h"
 #include "fbobject.h"
 #include "framebuffer.h"
@@ -867,6 +868,8 @@ _mesa_drawbuffers(struct gl_context *ctx, struct gl_framebuffer *fb,
          }
       }
    }
+
+   _mesa_update_valid_to_render_state(ctx);
 }