Hook up the flags to allow read-only ppGTT mappings for gen8+

v2: Include a selftest to check that writes to a readonly PTE are
dropped
v3: Don't duplicate cpu_check() as we can just reuse it, and even worse
don't wholesale copy the theory-of-operation comment from igt_ctx_exec
without changing it to explain the intention behind the new test!
v4: Joonas really likes magic mystery values
Signed-off-by: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712185315.3288-2-chris@chris-wilson.co.uk
(backported from commit 250f8c81)

CVE-2019-0155

[tjaalton: backport to i915_bpo
 - ggtt_vm doesn't exist, use ggtt->base
 - dev_priv doesn't have gtt, use ggtt instead]
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>