drm/radeon/kms: disable the r600 cb offset checker for linear surfaces

There are too many strange corner cases triggered in old userspace
drivers out there to that it's nearly impossible to not break some
obscure app.

Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>