st/mesa: set stObj->lastLevel in guess_and_alloc_texture

Fixes lockups/asserts with depthstencil-render-miplevels tests and r600g.
Should also fix https://bugs.freedesktop.org/show_bug.cgi?id=50033

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/state_tracker/st_cb_texture.c

@@ -400,6 +400,8 @@ guess_and_alloc_texture(struct st_context *st,
                                  ptLayers,
                                  bindings);
 
+   stObj->lastLevel = lastLevel;
+
    DBG("%s returning %d\n", __FUNCTION__, (stObj->pt != NULL));
 
    return stObj->pt != NULL;