agp/intel: restore cache behavior on sandybridge

This restores cache behavior for default AGP_USER_MEMORY as
uncached, and leave default AGP_USER_CACHED_MEMORY as LLC only.
I've seen different cache behavior on one sandybridge desktop CPU vs.
another mobile CPU. Until we figure out how to detect the real cache
config, restore back to the original behavior now.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>