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.
Instead of complicating the blit-path, let's just do the fallback-path here. That keeps the blit-path nice and optimistic.
This wasn't a problem until recently, because none of the compressed formats with fallback were copyable as per the OpenGL 4.6 Core spec, table 8.14. However, this changed when we introduced fallback for S3TC formats.
Fixes: 4f4e28fa ("mesa/st: implement fallback for bptc")