i965: Prefer using streaming reads from WC mmaps

i965: Prefer using streaming reads from WC mmaps

For buffer objects, where we primarily expect to be writing to them and
so already have a WC mmap (for !llc access) reusing the existing mmap
and keeping the buffer out of the CPU cache seems preferable.

Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Matt Turner <mattst88@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>