mesa: Handle updating texture state for buffer textures.

mesa: Handle updating texture state for buffer textures.

We have to skip some work that wants to look at texture images, since
buffer textures don't have any of that complexity.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>