st/mesa: fix blit-based GetTexImage for non-finalized textures

This fixes getteximage-depth piglit failures on radeonsi.

Cc: 11.1 11.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
(cherry picked from commit 3d956b4bc0)

src/mesa/state_tracker/st_cb_texture.c

@@ -2136,7 +2136,8 @@ st_GetTexSubImage(struct gl_context * ctx,
       goto fallback;
    }
 
-   if (!stImage->pt || !src) {
+   /* Handle non-finalized textures. */
+   if (!stImage->pt || stImage->pt != stObj->pt || !src) {
       goto fallback;
    }