drm/ast: switch to using CACHED by default for sysram

This fixes problems on ppc64 platforms, where we could end up using
a WC mapping for migrating BOs with memcpy, when really we want to
use cached memory.

Tested-by: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>