llvmpipe: flush resources for kms swrast path.

The kms_swrast path calls this callback via the dri2 paths,
not flushing caused artifacts when running inside a VM or on hw
in weston/gnome-shell.

Fixes: 6bbbe15a78 ("Reinstate: llvmpipe: allow vertex processing and fragment processing in parallel")
Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16715>

src/gallium/drivers/llvmpipe/lp_surface.c

@@ -174,6 +174,7 @@ static void lp_blit(struct pipe_context *pipe,
 static void
 lp_flush_resource(struct pipe_context *ctx, struct pipe_resource *resource)
 {
+   llvmpipe_flush_resource(ctx, resource, 0, true, true, false, "resource");
 }