i965: Assert that we don't use CPU write maps to non-coherent buffers.

i965: Assert that we don't use CPU write maps to non-coherent buffers.

Using CPU maps of non-coherent buffers can get us in a lot of trouble,
and WC maps are a reasonable alternative anyway.  Guard against shooting
ourselves in the foot by adding an assert, and comment.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matt Turner <mattst88@gmail.com>