mesa/st: do not blit when using compressed fallback

mesa/st: do not blit when using compressed fallback

If we're using the blit-path, we don't update the compressed image, which
will be needed if an application try to download the texture-image again
afterwards.

I previously fixed this for the memcpy-codepath, but we actually need to
fall back for the blit code-path as well. So let's move this early-out
earlier to catch both.

Fixes: 8f446322e1b ("mesa/st: do not use memcpy when using compressed fallback")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18342>