st/mesa: simplify lastLevel determination in st_finalize_texture

This fixes shader images where we always bind stObj->pt and not individual
gl_texture_images.

Roughly based on i965 commit 845ad266
which does a similar thing but for a different reason.

This fixes GL CTS assertion failures introduced by Ilia.

Cc: 18.0 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
(cherry picked from commit a4ba7cd6)