In st_finalize_texture() we were looking at the st_texture_object::
lastLevel field instead of the pipe_resource::last_level field to
determine which resource to store the mipmap in.

Then, in st_generate_mipmap() we need to call st_finalize_texture() to
make sure the destination resource is properly allocated.

These changes fix the broken piglit fbo-generatemipmap-formats test.