zink: fix descriptor buffer unmaps on screen destroy

descriptor buffer uses mapped buffers. mapping/unmapping buffers
uses a ctx in the function params, but at this time there is no ctx.
since the ctx is not actually used for unmapping descriptor buffers,
this can instead use a special buffer unmap function to avoid invalid access

Fixes: b06f6e00fb ("zink: fix heap-use-after-free on batch_state with sub-allocated pipe_resources")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27344>

src/gallium/drivers/zink/zink_context.c

@@ -173,6 +173,7 @@ zink_context_destroy(struct pipe_context *pctx)
    while (bs) {
       struct zink_batch_state *bs_next = bs->next;
       zink_clear_batch_state(ctx, bs);
+      bs->ctx = NULL;
       /* restore link as we insert them into the screens free_batch_states
        * list below
        */

src/gallium/drivers/zink/zink_descriptors.c

@@ -1501,7 +1501,7 @@ zink_batch_descriptor_deinit(struct zink_screen *screen, struct zink_batch_state
    }
 
    if (bs->dd.db_xfer)
-      pipe_buffer_unmap(&bs->ctx->base, bs->dd.db_xfer);
+      zink_screen_buffer_unmap(&screen->base, bs->dd.db_xfer);
    bs->dd.db_xfer = NULL;
    if (bs->dd.db)
       screen->base.resource_destroy(&screen->base, &bs->dd.db->base.b);

src/gallium/drivers/zink/zink_resource.c

@@ -2985,6 +2985,16 @@ do_transfer_unmap(struct zink_screen *screen, struct zink_transfer *trans)
    unmap_resource(screen, res);
 }
 
+void
+zink_screen_buffer_unmap(struct pipe_screen *pscreen, struct pipe_transfer *ptrans)
+{
+   struct zink_screen *screen = zink_screen(pscreen);
+   struct zink_transfer *trans = (struct zink_transfer *)ptrans;
+   if (trans->base.b.usage & PIPE_MAP_ONCE && !trans->staging_res)
+      do_transfer_unmap(screen, trans);
+   transfer_unmap(NULL, ptrans);
+}
+
 static void
 zink_buffer_unmap(struct pipe_context *pctx, struct pipe_transfer *ptrans)
 {

src/gallium/drivers/zink/zink_resource.h

@@ -44,7 +44,8 @@ zink_screen_resource_init(struct pipe_screen *pscreen);
 
 void
 zink_context_resource_init(struct pipe_context *pctx);
-
+void
+zink_screen_buffer_unmap(struct pipe_screen *pscreen, struct pipe_transfer *ptrans);
 void
 zink_get_depth_stencil_resources(struct pipe_resource *res,
                                  struct zink_resource **out_z,