drm/i915: Drop __rcu from gem_context->vm

It's been invariant since

    commit ccbc1b97
    Author: Jason Ekstrand <jason@jlekstrand.net>
    Date:   Thu Jul 8 10:48:30 2021 -0500

        drm/i915/gem: Don't allow changing the VM on running contexts (v4)

this just completes the deed. I've tried to split out prep work for
more careful review as much as possible, this is what's left:

- get_ppgtt gets simplified since we don't need to grab a temporary
  reference - we can rely on the temporary reference for the gem_ctx
  while we inspect the vm. The new vm_id still needs a full
  i915_vm_open ofc. This also removes the final caller of context_get_vm_rcu

- A pile of selftests can now just look at ctx->vm instead of
  rcu_dereference_protected( , true) or similar things.

- All callers of i915_gem_context_vm also disappear.

- I've changed the hugepage selftest to set scrub_64K without any
  locking, because when we inspect that setting we're also not taking
  any locks either. It works because it's a selftests that's careful
  (single threaded gives you nice ordering) and not a live driver
  where races can happen from anywhere.

These can only be split up further if we have some intermediate state
with a bunch more rcu_dereference_protected(ctx->vm, true), just to
shut up lockdep and sparse.

The conversion to __rcu happened in

commit a4e7ccda
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Oct 4 14:40:09 2019 +0100

    drm/i915: Move context management under GEM

Note that we're not breaking the actual bugfix in there: The real
bugfix is pushing the i915_vm_relase onto a separate worker, to avoid
locking inversion issues. The rcu conversion was just thrown in for
entertainment value on top (no vm lookup isn't even close to anything
that's a hotpath where removing the single spinlock can be measured).

v2: Rebase over the change to move the i915_vm_put() into
i915_gem_context_release().

v3: Trivial conflict against repainted shed.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210902142057.929669-9-daniel.vetter@ffwll.ch