drm/panfrost: Job should reference MMU not file_priv

For a while now it's been allowed for a MMU context to outlive it's
corresponding panfrost_priv, however the job structure still references
panfrost_priv to get hold of the MMU context. If panfrost_priv has been
freed this is a use-after-free which I've been able to trigger resulting
in a splat.

To fix this, drop the reference to panfrost_priv in the job structure
and add a direct reference to the MMU structure which is what's actually
needed.

Fixes: 7fdc48cc

 ("drm/panfrost: Make sure MMU context lifetime is not bound to panfrost_priv")
Signed-off-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220519152003.81081-1-steven.price@arm.com